The six common forms are not interchangeable. Lot, parking, an existing shell and who will live in it usually pick the form before taste does. This is a recommendation to take to planning, not a yes.
Six formsone of them is usually forced by the lotDetached, attached, garage conversion, basement, above-garage, prefab. Which are permitted, and in what size, is entirely a local question. The homepage already says so.
Answer five prompts about the lot, parking, any existing shell, intended use and privacy. The tool names a form and the case that would make it the wrong form. It will not say you can build.
A garage you can give up usually points at a conversion (cost lines live on HyreGarage). A tight lot that must keep parking usually points at attached or above-garage. A wide lot with a privacy goal is the actual case for detached. Take the result to planning.
Five prompts, then a form
“I don’t know” is a valid state. Nothing is emailed.
—Form to take to planning
—Why this form, on these answers
—Wrong if
—What this is not
What this assumed—
A selector of form, never a determination that your parcel qualifies. HyreADU does not design or permit ADUs.
Three measurements and one statute usually pick the form before you do
People approach this as a design question and it is mostly not one. In the great majority of cases the form is decided by a ceiling height, a window opening, the distance to a sewer, and a rule about parking that works in the opposite direction to what everyone expects.
The three measurements, in the order they eliminate options. First, ceiling height, because a space that cannot legally be a habitable room cannot be a dwelling and no amount of drawing changes that.
Second, the escape and rescue opening, because a sleeping room needs one and retrofitting it into a basement wall or a garage gable is structural work with a real price.
Third, distance to the sewer connection and the electrical panel, because that is what separates a straightforward conversion from a trenching project across a garden.
Each of those takes a tape measure and an afternoon. None requires an architect, a survey or a fee, and doing them first is the difference between a designer’s first meeting being productive and it being an expensive way to find out your basement is six foot ten.
That discovery, made after the drawings are paid for, is one of the more common and more avoidable ways an ADU project ends.
Then the statute, and here is the counter-intuitive part. The objection homeowners brace hardest for on a garage conversion is parking — the belief that the city will make them replace the spaces the garage was providing, which on a small lot is often impossible and so kills the idea before it starts.
This is the single most common objection people brace for, and in California it is not available to the agency. It cannot make you replace the parking the garage was providing.
The words themselves (Gov. Code § 66314(d)(11)): "When a garage, carport, covered parking structure, or uncovered parking space is demolished in conjunction with the construction of an accessory dwelling unit or converted to an accessory dwelling unit, the local agency shall not require that those offstreet parking spaces be replaced."
Read what that does to the decision. The form most people rule out first is, on this axis, the one with the strongest statutory protection.
It also tends to have the shortest utility runs, because a garage attached or adjacent to the house is near the existing services, and an existing slab and shell it can reuse.
The form people rule out last — a new detached cottage in the back garden — is the one with the longest trench, the new foundation, and the least existing anything.
None of which means a conversion is the right answer. A garage has a low slab-to-plate height, no insulation, a floor that slopes to a door, and often no proper footing under the perimeter. Those are real problems and they cost real money.
The point is only that the reason people usually reject it is not one of them, and the reasons they usually reject a detached unit rarely include the ones that will actually bite.
What this tool is. It is a way of naming a form to take to a planning counter, not a permission. HyreADU does not design, permit or build accessory dwelling units.
It does not appraise, lend, invest, let property or prepare taxes, it does not rank or refer contractors, and it takes no referral fee from anyone who does any of those things.
A form is a zoning question and a construction-code question at the same time, answered by two different departments, and neither of them is us.
The six forms, and what actually decides each one
The last column is the one to read. It is not the advantage or the disadvantage — it is the single fact that most often makes the form impossible, which is the thing to check before anything else.
Form
What it reuses
What it still needs
The fact that most often kills it
Detached new build
Nothing. A clear pad and open ground.
Foundation, full envelope, its own utility runs, and enough yard left after setbacks and lot coverage.
Trench length and a lot-coverage limit. Two facts that live in different documents and are usually checked in the wrong order — coverage first, because it is free to check.
Attached (shares a wall)
One wall, and proximity to existing services.
Fire separation at the shared assembly, a separate entrance, and its own foundation for the new footprint.
The shared wall assembly. Separation requirements between dwelling units are a construction-code matter and can be more work than the wall saves.
Garage conversion
Slab, walls, roof, and short utility runs.
Insulation, a real floor, egress, ceiling height, and usually a proper perimeter footing the slab never had.
Slab-to-plate height, and whether the existing footing can carry a habitable structure. Not parking, in states that have removed that objection.
Basement conversion
Structure, envelope, and the shortest utility runs of any form.
Ceiling height, an escape and rescue opening, moisture control, and often a separate entrance cut into a foundation wall.
Ceiling height. This is the figure that decides whether a basement or an above-garage space can become a dwelling at all, and it is the one most often discovered after money has been spent on drawings. Measure it first.
Above a garage
The garage footprint and its foundation, if it is adequate.
A structure capable of carrying a floor load it was never designed for, a stair, egress from the upper level, and a height envelope that permits two stories.
The existing garage structure. A single-story garage is rarely engineered to carry a dwelling above it, and the strengthening is not a small line.
Prefab or modular
Factory labor and a compressed on-site program.
Everything a detached unit needs — foundation, utilities, crane or truck access — plus a delivery route wide enough for the module.
Access. A module that cannot reach the pad is not a building, and this is discovered late because nobody measures the side return until the lorry is booked.
Our analysis of how these forms differ, assembled from the code and statutory material sourced elsewhere on this page. It is not a cost ranking: we publish no ADU price and hold no dataset of bids, so we cannot and do not tell you which of these is cheaper for you.
The measurement that ends more basement and above-garage projects than anything else
Seven feet. It is a model-code figure, it is not negotiable by design, and it is the single cheapest thing you can check today.
Four numbers from one code section, on one axis. The gap between the first bar and the second is the whole reason a plan will try to put the bathroom under the low part of a roof.International Code Council, International Residential Code, IRC R305.1, ceiling height, as retrieved from a jurisdiction-published handout. Retrieved 2026-09-05.
What the code says
IRC R305.1, ceiling height: "Habitable space, hallways and portions of basements containing these spaces shall have a ceiling height of not less than 7 feet (2134 mm). Bathrooms, toilet rooms and laundry rooms shall have a ceiling height of not less than 6 feet 8 inches (2032 mm)."
And for sloped ceilings: "For rooms with sloped ceilings, the required floor area of the room shall have a ceiling height of not less than 5 feet (1524 mm) and not less than 50 percent of the required floor area shall have a ceiling height of not less than 7 feet (2134 mm)." That second sentence is what makes an attic or a room under a pitched roof workable at all — the whole ceiling does not have to be at seven feet, but half the required floor area does, and no part of the counted area may be under five.
And for the rest of a basement: "Portions of basements that do not contain habitable space or hallways shall have a ceiling height of not less than 6 feet 8 inches (2032 mm)." Which is why a basement can be perfectly serviceable as storage and still fail as a dwelling: the two uses are held to different numbers, and the number that applied when it was built was the lower one.
Model-code caveat, stated every time. The IRC is a model document with no legal force until a jurisdiction adopts it, and adopting jurisdictions routinely amend it. Confirm every figure with the authority that will inspect the work.
Neither retrieved document states its IRC edition on its face. The numbering matches the 2021 structure and identical language appears in a state adoption of the 2018 edition, so we present these as model-code figures without an edition label rather than assert one we did not verify.
ICC’s own code library returned 403 to automated retrieval on 2026-09-05. The text below comes from jurisdiction-published copies, named in the sources.
How to measure it properly, in ten minutes
Measure to the finished ceiling you will end up with, not to the joists. This is where the optimism enters.
An open basement measuring seven foot two to the underside of the joists is not a seven-foot room once it has a ceiling finish, and it is certainly not one once a duct, a drain or a beam crosses it.
Then measure to the underside of the lowest obstruction. Ductwork, a waste pipe, a steel beam, a boiler flue. Whatever is lowest is the number that governs the space beneath it, and moving services is a cost line, not a detail.
And measure the floor build-up you have not added yet. A basement slab often needs a damp-proof membrane, insulation and a screed or a floating floor before it is a dwelling floor.
Every inch of that comes off the ceiling height. Two or three inches is ordinary, and two or three inches is frequently the whole margin.
Do all three before you commission anything. If the answer is close to seven feet rather than comfortably above it, that is a conversation to have with a designer on day one, framed as "can this be done at all", not on day sixty as "why has this become expensive".
What people try, and what it costs
Digging down. Lowering a basement floor is possible and it is structural work, because the existing footings were not designed to be undermined.
It is engineered, permitted, and among the more expensive things you can do to a house per square foot of benefit. It is sometimes the right answer. It is never a small change of plan.
Raising the roof over a garage. Also possible, also a genuine structural project, and it interacts with the height envelope: a taller building may exceed what the zoning permits for a detached accessory structure even where the construction is sound. Two rules, two departments, one project.
Redesignating rooms. The wet-room figure is lower — bathrooms, toilet rooms and laundry rooms at 6 ft 8 in — so a plan can sometimes put the low corner where the shower goes.
This is a real and legitimate design move, not a loophole, and it is a good early question for a designer who has done conversions.
Giving up on this space and moving the unit. Which is often the honest answer and is rarely the first one offered by anyone paid to convert basements.
The window that has to satisfy three dimensions at once
A sleeping room needs an emergency escape and rescue opening. Most existing basement and garage openings fail it, and they fail it on the area, not on the dimensions people check.
This chart is arithmetic on the code’s own minimums, not a survey of installed windows. It exists to make one point: the two dimensions and the area are three separate tests, and passing two of them is not passing.International Code Council, International Residential Code, IRC R310, emergency escape and rescue openings, as retrieved from a jurisdiction-published Code and Commentary excerpt. Retrieved 2026-09-05.
What is required, and where. IRC R310, emergency escape and rescue openings: "Basements, habitable attics and every sleeping room shall have not less than one operable emergency escape and rescue opening". Basements, habitable attics and every sleeping room. Not every room — the sleeping rooms and the basement.
The area. "Emergency escape and rescue openings shall have a net clear opening of not less than 5.7 square feet (0.530 m2). Exception: The minimum net clear opening for grade-floor emergency escape and rescue openings shall be 5 square feet (0.465 m2)." So 5.7 square feet of net clear opening, with a 5 square foot exception at grade level.
The dimensions. "The minimum net clear opening height dimension shall be 24 inches (610 mm). The minimum net clear opening width dimension shall be 20 inches (508 mm). The net clear opening dimensions shall be the result of normal operation of the opening." A minimum of 24 inches of clear height and 20 inches of clear width.
Now the arithmetic that catches people. A sleeping room needs an opening an occupant can get out of and a firefighter in breathing apparatus can get in through, worked by normal operation.
The three dimensions have to be satisfied at the same time: a window that meets the 24-inch height and the 20-inch width can still fail the 5.7 square feet, and most do.
Multiply the two minimum dimensions together: 24 by 20 inches is 480 square inches, which is about 3.33 square feet — well under the 5.7 required.
That is HyreADU multiplication on the code’s own figures, and it is the single most useful thing on this page for anyone with a basement.
An opening that satisfies both minimum dimensions can still fail the area requirement by a wide margin, and a window ordered on the two dimensions alone is a window that will not pass.
And the sill height. "Emergency escape and rescue openings shall have the bottom of the clear opening not greater than 44 inches (1118 mm) above the floor." That 44 inches is measured to the bottom of the actual clear opening, not to the sill height of the rough opening.
The ICC Commentary is explicit about it, and it is a common and expensive misreading.
Why this decides the form. In a basement, providing a compliant opening usually means cutting a larger hole in a foundation wall and excavating a window well outside it, with a guard and sometimes a ladder.
That is engineering, excavation and drainage — a project inside the project. In a garage conversion, the gable or the side wall may take a window easily, or the existing structure may not allow the header.
In a new detached unit the question does not arise, because the opening is drawn in from the start. This is one of the clearest cases where a construction-code requirement, not a preference, selects the form.
What we are not telling you. Nothing on this page is instruction to cut an opening in a structural wall, and it should not be read as one.
Modifying a foundation or a load-bearing wall is engineered, permitted work. The value of knowing the numbers is that you can ask a designer the right question early, not that you can answer it yourself.
California treats the forms differently, and the differences are worth money
The often-quoted "800 square feet, sixteen feet, four-foot setbacks" floor is real, but it is assembled from three separate provisions and it does not apply uniformly across the forms. Quoting it as one rule is how people apply it to the wrong ADU.
What the statute forbids a local agency from imposing
The often-quoted "800 square feet, 16 feet, four-foot setbacks" floor is real, but it does not live in one section. It is assembled from § 66321(b)(3), § 66321(b)(4)(A) and § 66323(a)(2), and quoting it as a single rule is how people end up applying it to the wrong ADU type.
§ 66321(b)(1): A minimum square footage requirement for either an attached or detached accessory dwelling unit that prohibits an efficiency unit.
§ 66321(b)(2): A maximum square footage below 850 square feet, or below 1,000 square feet for a unit with more than one bedroom.
§ 66321(b)(3): Any requirement that prevents construction of a unit with at least 800 square feet of interior livable space and with four-foot side and rear yard setbacks.
§ 66321(b)(4)(A): A height of 16 feet for a detached accessory dwelling unit on a lot with an existing or proposed single family or multifamily dwelling unit.
§ 66321(b)(4)(B): A height of 18 feet for a detached unit near transit, plus an additional two feet to accommodate a roof pitch.
§ 66321(b)(4)(D): A height of 25 feet, or the local limit applying to the primary dwelling, whichever is lower, for an ADU attached to a primary dwelling.
Notice how the height limbs differ by form. A detached unit, a detached unit near transit, and an attached unit are three different numbers in three different subparagraphs.
A homeowner who has read "sixteen feet" and is planning an attached two-story unit has read the wrong limb, and the one that applies to them is more generous, not less.
The conversion setback exemption, which is easy to miss
Gov. Code § 66314(d)(7): "No setback shall be required for an existing living area or accessory structure or a structure constructed in the same location and to the same dimensions as an existing structure that is converted to an accessory dwelling unit"
This is a substantial advantage and it is specific to conversions. An existing garage sitting on the rear boundary, which could never be built there today, does not have to move to become an ADU — nor does a replacement structure in the same location and to the same dimensions.
On a small urban lot where four-foot setbacks would eat most of the buildable area, that provision can be the difference between a viable unit and none.
And a related one: "A local agency shall not require, as a condition for ministerial approval of a permit application for the creation of an accessory dwelling unit or a junior accessory dwelling unit, the correction of nonconforming zoning conditions." (Gov. Code § 66323(a) and following).
A local agency cannot use the ADU application as the occasion to make you fix unrelated nonconforming conditions on the property — a common and expensive form of scope creep at the counter.
Fire sprinklers, and what the statute does with them
Gov. Code § 66314(d)(12): "Accessory dwelling units shall not be required to provide fire sprinklers if they are not required for the primary residence. The construction of an accessory dwelling unit shall not trigger a requirement for fire sprinklers to be installed in the existing primary dwelling."
Two separate protections in one sentence. The unit need not have sprinklers if the house does not, and — the more valuable half — building the ADU does not trigger a retrofit requirement in the existing house.
That second clause exists because retrofitting sprinklers into an occupied dwelling is expensive enough to end a project on its own, and the legislature evidently knew it.
This is a form question because it is not neutral across forms. Where a conversion or an addition might otherwise be treated as work triggering an upgrade to the primary dwelling, the statute forecloses it. Confirm how your jurisdiction applies it, because adoption and amendment are local.
The pre-approved plan route only reaches one form
Gov. Code § 65852.27, AB 1332 (Carrillo, 2023), effective 2025-01-01: A complete application for a DETACHED ADU that uses a plan preapproved in the current triennial California Building Standards Code cycle, or a plan identical to one the agency already approved in that cycle, must be approved or denied ministerially within 30 days.
By its terms that clock does not reach attached ADUs, garage conversions or JADUs unless a local program separately covers them.
Read the word "detached". A thirty-day ministerial clock is materially faster than the sixty-day general one, and it is available only to a detached unit using a plan already pre-approved in the current code cycle.
If your city runs such a program and one of its plans suits your lot, that is a genuine reason to choose the detached form that has nothing to do with design — and it is invisible unless you go looking for the program.
And a reason it might not suit you. A pre-approved plan is somebody else’s building. It comes with fixed dimensions, a fixed layout and a fixed appearance. The speed is real and so is the constraint, and only you can weigh them.
What to establish before you commit to a form
The first four you can do yourself, today, for nothing. The last three are counter questions, and they are short.
Measure the ceiling height to the lowest obstruction, in every candidate space
Basement, garage, above-garage void. Write down three numbers: to the joists, to the lowest duct or pipe, and your estimate of the floor build-up you will add. The governing number is the smallest one after the build-up comes off.
Measure the existing openings and compute their net clear area
Height times width of the opening you can actually get through, in inches, divided by 144. Compare with 5.7 square feet. Do the sill height too, to the bottom of the clear opening.
Walk the sewer and panel route with a tape
From each candidate location to the cleanout or the main, and to the electrical panel. Note anything in the way: a driveway, a mature tree, a patio, a neighbor’s wall. Trench length and what is on top of it are most of the utility cost difference between the forms.
Check lot coverage and setbacks before you fall in love with a detached unit
Both are published in your zoning ordinance and both are free to look up. Coverage is the one that quietly rules out a detached building on a lot that visually has plenty of room.
Ask the planning counter which forms are ministerial here, and on what clock
Not "can I build an ADU" — ask which forms get ministerial approval, what the decision deadline is, and whether there is a pre-approved plan program. Three specific questions get three specific answers.
Ask both utilities, separately, what they will charge for a connection
Water and sewer, and the electricity provider about panel capacity. Get it in writing. This is the line that most often ends a project after approval, and it is the one no builder controls.
Ask a designer who has permitted this form in this city what the plan check asked for
The specific corrections their last set of drawings drew, and how long the cycle took. That answer tells you more about your real timeline than any published deadline.
The distinction that changes which rules apply to you
A junior accessory dwelling unit is not a small ADU. It is a separate creature under a separate article, and several of the protections people rely on do not carry across.
Where they differ in ways that matter
Different statutory article. The JADU decision clock sits at Gov. Code § 66335. The parallel 60-day decision for junior accessory dwelling units. Reading the ADU sections and assuming they govern a JADU is a category error, and a common one.
Owner-occupancy. We looked for a sunset or expiry on that prohibition and did not find one, and we did not retrieve the separate JADU article (§§ 66333 and following), which has its own owner-occupancy treatment.
Do not read this as covering a JADU. We state the prohibition for ADUs and we do not extend it to JADUs, because we did not retrieve the article that would let us.
Fees. The impact-fee threshold differs by unit type: 750 square feet for an ADU and 500 for a JADU. Two numbers, two creatures.
The pre-approved plan clock. By its terms the 30-day route reaches detached ADUs. It does not reach JADUs unless a local program separately provides for them.
Why the distinction is a design decision, not a definition
A JADU is created within the walls of the existing house. That means the shortest possible utility runs, no new foundation, and often no change to the exterior at all — which is also why it is the form most likely to be treated permissively.
It also means giving up part of your house. A bedroom, usually, plus circulation. That is a real cost that never appears in a budget, and it is paid in space rather than money.
The smaller fee threshold cuts both ways. A JADU is small by nature, so the lower threshold is easier to stay under than it looks — but a JADU at 501 square feet has crossed a line an ADU at 501 has not.
A unit at 749 square feet of interior livable space and a unit at 751 can differ by five figures of park, traffic and school charges before anyone lifts a hammer.
And it is the form that most often needs no builder at all, in the sense that the work is closer to a bathroom and kitchenette fit-out than to a construction project. That changes who you should be talking to, and it changes the shape of the bids you will get.
We publish the ADU provisions we retrieved and are explicit about the JADU article we did not. If your project is a JADU, treat every ADU-labeled statement on this site as needing separate confirmation.
The same objections, removed in state after state
Seven states, seven drafting styles, one pattern. Each entry below was retrieved individually from that state’s own legislature or code site; states we could not retrieve were dropped and are named at the end.
California
Gov. Code §§ 66310–66342
Ministerial approval on a 60-day clock with deemed approval, no replacement parking, no owner-occupancy requirement, and no fire sprinklers triggered in the existing house.
Washington
RCW 36.70A.681 (HB 1337, 2023)
A hard 50 per cent cap on ADU impact fees relative to the principal unit, and detached units must be permitted. The remaining subsections were retrieved only as paraphrase, so this site quotes only the two sentences above.
Montana
MCA 76-2-345 (SB 528, ch. 502, L. 2023)
One ADU by right, sized at 75 per cent of the house or 1,000 square feet, whichever is less. If a municipality misses the compliance deadline its own regulations are void.
Colorado
C.R.S. § 29-35-103 (HB24-1152)
No new off-street parking, no owner-occupancy requirement, no restrictive design standard — and homeowners association covenants banning ADUs are void as a matter of public policy, including ones adopted before the law, subject to a defined "reasonable restriction" carve-out.
Arizona
A.R.S. § 9-461.18 (HB 2720, 2024, as engrossed)
Municipalities over 75,000 must permit at least one ADU by right, may not require additional parking, a familial or employment relationship between occupants, matching exterior design, or a restrictive covenant — and a city that missed the deadline loses its limits entirely.
Retrieved as the House Engrossed version. We did not retrieve the chaptered session law, so it is cited as engrossed.
Maine
30-A M.R.S. § 4364-B (LD 2003 and later amendments)
A statutory amnesty for units built without municipal approval, a 190-square-foot minimum size, one ADU exempt from density calculations, no additional parking, and no owner-occupancy.
Oregon
ORS 197A.425 (formerly ORS 197.312(5)–(6))
At least one ADU for each detached single-family dwelling in cities over 2,500 and counties over 15,000 inside urban growth boundaries. Reasonable local siting and design regulation excludes owner-occupancy requirements and additional off-street parking.
Retrieved from a private reproduction of the Oregon Revised Statutes, not from oregonlegislature.gov, which returned no content. Treated as second-tier sourcing and not quoted verbatim on this site.
Seven states, seven different drafting styles, and the same three objections removed in most of them: replacement parking, owner-occupancy, and a requirement that the unit match the house.
Several also attach a self-executing penalty — California deems the application approved, Arizona removes the city’s limits entirely, Montana voids the regulations — which tells you legislatures expected local resistance and drafted against it.
Vermont, New Hampshire, Rhode Island and Utah were on our list and are not here.
We could not retrieve a statute, bill text or state agency page for any of them on the retrieval date, so this site says nothing about them.
What this selector cannot do
Six specific blind spots. Each is something a reader might reasonably expect a form selector to handle, and it does not.
It cannot tell you what your zone allows. Form is a zoning question answered by an ordinance we have not read, on a parcel we cannot see.
Setbacks, lot coverage, height, separation between structures and overlay districts all vary by parcel, and any one of them can make a form impossible on your lot while permitting it two streets away.
It cannot price the forms against each other. We publish no ADU price and hold no dataset of bids or quotes.
The intuition that a conversion is cheaper than a new build is often right and is sometimes badly wrong — a conversion that needs underpinning, a new footing and a dug-out floor can exceed a straightforward detached unit on level ground.
It cannot assess your existing structure. Whether a garage can carry a story, whether a footing can take a habitable load, whether a slab is sound — those are questions for a structural engineer who has been on site, and they are the questions that most often change the answer.
It cannot see the utility route. Distance to the sewer main, the condition of the lateral, spare panel capacity, gas sizing. These are the variables that separate two otherwise identical projects, and they are knowable only from your parcel and your utilities.
The code figures are model-code figures. The IRC is a model document with no legal force until a jurisdiction adopts it, and adopting jurisdictions routinely amend it. Confirm every figure with the authority that will inspect the work.
Neither retrieved document states its IRC edition on its face. The numbering matches the 2021 structure and identical language appears in a state adoption of the 2018 edition, so we present these as model-code figures without an edition label rather than assert one we did not verify.
And the statutory material is Californian: the shape of the questions transfers to other states, the citations do not.
And a permit is not a building. These are reported APR rows, not a physical census of construction.
An unmatched permit may be a unit still under construction, an abandoned project, a unit completed without a reported certificate, or a project whose identifier changed between the permit row and the completion row.
The direction is robust; the exact rate is a reporting artefact as much as a construction one. Choosing the right form well does not, by itself, get a unit built.
The two things that most often stop a project after approval are financing and the utility connection charge, and neither of them is a form question at all.
When the answer is that none of these forms is right
A tool that can only ever recommend building something is not a tool, it is a funnel. This one is allowed to say no, and it is worth saying when the no is honest.
Name the conflict. Nearly every “which ADU should I build” guide is published by somebody who earns when one gets built. HyreADU does not design, permit or build accessory dwelling units.
It does not appraise, lend, invest, let property or prepare taxes, it does not rank or refer contractors, and it takes no referral fee from anyone who does any of those things.
That is what allows these tools to conclude that the numbers do not work, that the answer is to keep the garage, or that the honest next step is a question at the planning counter rather than a deposit.
The cases where no form works. A basement at six foot eight with a duct across it, on a lot with no room for a detached pad after coverage, with a garage on a slab that has no perimeter footing.
That is not a design brief, it is three separate structural projects wearing one name. Somebody will happily quote for it. Whether you should accept is a different question.
The intermediate answers people forget. Waiting for a pre-approved plan program to open in your city. Building smaller and staying under a fee threshold.
Converting the space to a home office without a kitchen, which is not a dwelling and is a far simpler piece of work, if what you actually need is a room rather than a unit. Doing nothing for two years and keeping the money.
And the one question worth more than the form. Before choosing among six shapes, ask what the unit is for.
A rental, a parent, an adult child and a studio are four different briefs with four different privacy, access and kitchen requirements, and the form that suits one can be actively wrong for another.
An above-garage unit with a steep external stair is a reasonable rental and a poor home for someone who is eighty.
That constraint outranks every measurement on this page, and it is the one people leave until last.
How this calculator works
The selector is a short decision tree, not a score. It reads the answers in this order:
A garage you can give up → conversion, with a pointer at HyreGarage for the cost lines.
A basement → basement ADU, gated on egress, ceiling height and moisture.
A garage you must keep → above-garage, gated on whether the structure can take the load.
A tight or unmeasured lot → attached, because a new pad is the thing tight lots fail first.
A wide lot plus a high privacy goal → detached.
An office use → a reminder that a kitchen is what turns a studio into a dwelling under many local definitions.
Otherwise → detached, which is what most people mean by “an ADU.”
Every result includes a “wrong if” and a hard line that this is not a determination. The engine is tested to recommend conversion when a garage can be given up, and to refuse eligibility language in every case.
What each input means
Inputs on this tool, in the order they appear on the form.
Input
What it is actually asking
Yard / remaining pad
Have you measured the rectangle inside rear and side setbacks, after coverage? “I have not measured” is the honest default and pushes the result toward attached, because detached needs a pad you have not shown exists.
Parking
Whether you must keep an off-street stall. Conversion of a required garage is how a cheap envelope becomes an illegal parking deficiency.
Existing volume
Garage or basement you would actually convert. “None” means you are adding a building or an addition.
Intended use
Rental, family or office. Use changes layout, entrance, parking and, in some jurisdictions, what is permitted at all. Settle it before design.
Privacy
A preference. It only wins when the lot can hold a detached envelope. Preference does not move a setback.
Worked examples
Including one where the naive answer misleads, which is the example most calculators leave out.
A two-car garage, parking not required to stay
Existing = garage, parking = can give up. Result: garage conversion, with the warning that parking lost can still be the expensive line, and that conversion cost is canonical on HyreGarage. The envelope exists. The work is making it a dwelling.
A small lot that must keep the garage
Lot = tight, parking = must keep, existing = garage. Result: above-garage if the structure can take the load. Conversion is the wrong form because the stall has to stay. Detached is the wrong form because there is no pad. The remaining question is structural, not aesthetic.
The one where “I want a backyard cottage” misleads
Lot = I have not measured, privacy = high, use = rental. A cottage drawing is what the internet will sell you. This tool names attached, because you have not shown that a pad exists.
The naive answer (detached, because that is the picture) is how people pay for drawings of a building the setbacks will not hold. Measure coverage and setbacks, or open the size-envelope tool, before you commission a cottage.
What changes the result
Parking is the switch that turns a garage from an opportunity into a constraint. The same garage is a conversion if the stall may go, and an above-garage candidate if the stall must stay.
An unmeasured lot is treated as tight. That is deliberate. Detached is recommended only when you claim a wide pad or when nothing else fits. Claiming a cottage without a measurement is the failure mode this page exists to catch.
Office use without a kitchen may not be an ADU. That can be good news (fewer dwelling rules) or a trap (you cannot later rent it as a dwelling without a new permit). The selector says so rather than pretending every backyard building is an ADU.
Local considerations
Which forms are permitted is a zoning question, layered on state law where a state has legislated. California’s ADU/JADU statutes are the example people quote nationally; they are California law. Oregon, Washington, Colorado and others have their own statutes.
Plenty of cities still treat a second dwelling as a variance. Two neighboring cities can permit different buildings on identical lots. The homepage already says this. The selector does not look up your zone.
When not to use this
Do not treat the named form as permission. Take it to planning as a question: “Is this form permitted on this parcel, at what size, with what parking?”
Do not use it as a structural opinion. Above-garage and basement results are gated on load, egress, ceiling height and moisture, none of which this page can see.
Do not skip the feasibility checker. Form is step two. Eligibility is step one, and even that tool will not say yes.
The one the lot, the parking rule and the existing structures will actually hold, among the forms your zone permits. Taste comes after that. This selector ranks form from those constraints. It does not know your ordinance.
What is the difference between an ADU and a JADU?
A junior ADU is a local-law category (widely used in California) for a smaller unit, often converted from existing space, with its own rules on size, parking and owner-occupancy. The labels are not interchangeable. Ask planning which word they will put on your application.
Is a garage conversion an ADU?
Sometimes. Sometimes it is a JADU. Sometimes it is a conversion that is not a dwelling. That is a planning-department label, which is why the feasibility tool refuses to say yes and why conversion cost is canonical on HyreGarage.
Is prefab cheaper?
The factory portion can be more predictable. Site work, foundation, crane or delivery access, utilities, permitting and connection remain a conventional construction project on your lot. The comparison table on the homepage is relative, not priced.
Can I put an office in the backyard instead?
If it has no kitchen and is not a dwelling under local definitions, it may not be an ADU at all. That can be simpler, or forbidden, depending on the zone. If you later want it to rent as a dwelling, adding a kitchen is a different permit. Decide that now.
What if I must keep the garage?
Then conversion is the wrong form. Above-garage keeps the parking and uses the footprint twice, if the existing structure and foundation can take the load. That is a structural question, not a sketch. Attached is the other way to avoid a new pad.
Does this mean my lot qualifies?
No. Nothing on this site establishes eligibility. Open the feasibility checker and then call planning.
Which form is most private?
Detached, with its own entrance, envelope and (usually) longer run from the house. Privacy is a reason to pick detached only if the pad exists after setbacks and coverage.
Which form is usually fastest on site?
Conversions and factory-built modules often spend less time as a hole in the yard. Review and utility scheduling still dominate, and they do not care how the walls arrived.
Does HyreADU design ADUs?
No. This is a selector of form. A designer who has permitted that form in your city is the next conversation.
Sources and methodology
Figures dated 26 August 2026. Last reviewed .
Homeowner guidance on accessory dwelling units (AARP Livable Communities, retrieved 2026-08-26. Cited for ADUs as a housing and ageing-in-place form, not as a catalog of what your zone allows.)
Adopted state and local building codes (International Code Council, retrieved 2026-08-26. Egress, ceiling height, fire separation and energy are construction rules under the adopted code. Form is also a zoning question.)