Three ceilings: remaining lot coverage, the rectangle inside setbacks, and the maximum floor area your ordinance states. The binding one is the smallest. Unknowns stay unknown. This is not a stamp.
min()of coverage remaining, pad, zoning maxGeometry plus a number you copied from the ordinance. We do not look up your zone. A missing zoning max is often the binding number, which is why the form lets it stay at 0.
Type lot area, maximum lot coverage as a percent, existing coverage (house, garage, patio, shed), the width and depth of the rectangle that remains after rear and side setbacks, and any maximum ADU floor area the ordinance states.
The tool reports remaining coverage, pad area, and which of those is binding.
On a worked 6,000 sq ft lot at 40% coverage with 1,800 sq ft already built, a 30 × 40 ft pad and an 800 sq ft zoning cap, remaining coverage is 600 sq ft and coverage is the binding constraint. It will not say you may build 600 sq ft.
Three ceilings, then the minimum
Leave a field at 0 if you have not looked it up. Nothing is emailed.
—Binding constraint
—Lot coverage remaining
—Pad after setbacks
—Zoning max
What this assumed—
min(remaining coverage, pad, zoning max). Numbers you typed. Not a determination you may build. HyreADU does not survey lots.
The binding constraint is whichever number is smallest, and most people compute one
Almost everyone arrives with a single figure in mind — usually a zoning maximum they read somewhere, or the 800 square feet they have heard is protected. A size envelope is not one number. It is the minimum of several independent ceilings, and the one that binds is rarely the one being thought about.
The three the calculator takes. Remaining lot coverage (what the ordinance allows minus what is already built), the rectangle that survives after rear and side setbacks, and any maximum floor area the ordinance states.
The tool reports all three and names the smallest. That is deliberately mechanical arithmetic on numbers you supply: it does not know your zone and will not look it up.
The three it cannot take, which bind just as hard. The statutory floor that overrides a local maximum in some cases but not others.
The 750 square feet of interior livable space at which impact fees switch on, which is a money ceiling rather than a legal one and is more consequential than either.
And the habitability minimums — ceiling height and emergency egress — which do not cap the building at all but decide whether a given space inside it counts as habitable.
Why the order of discovery matters. Coverage and setbacks are usually checked first because they are geometric and therefore feel like the real constraint.
In practice the fee cliff changes more budgets than either, and the ceiling height in an existing basement or above-garage space kills more schemes outright than any zoning number.
Both of those are discovered late, and late is what makes them expensive.
A note on what this page is not. There is no national table of ADU size caps here, and there will not be one.
Size caps are local, several states have moved them recently, and a table would be wrong on arrival for most readers.
The zoning maximum stays a field you type, and leaving it at zero — meaning “I have not looked it up” — is a legitimate state that the result will say out loud.
The statutory floor is real, and it is assembled from three provisions, not one
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.
Provision
What a local agency may not impose
What its absence would mean
§ 66321(b)(1)
A minimum square footage requirement for either an attached or detached accessory dwelling unit that prohibits an efficiency unit.
Without it, a minimum-size rule could be set high enough to prohibit a studio. The provision exists because a floor area minimum is a perfectly ordinary-looking standard that functions as a ban on the smallest and cheapest units.
§ 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.
Without it, a maximum of, say, 600 square feet would be the binding number on every lot in the jurisdiction regardless of geometry. Note the two different figures: 850, and 1,000 for more than one bedroom. A scheme that gains a bedroom gains 150 square feet of protected ceiling with it.
§ 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.
This is the “800 square feet with four-foot setbacks” everyone quotes, and its actual form is a prohibition on any requirement that prevents it — coverage, setbacks, floor-area ratio, open space, whatever the mechanism. Without it, a stack of individually reasonable standards could combine to leave no buildable envelope at all.
§ 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.
Without it, a 12-foot height limit in a residential zone would make a detached unit with any usable roof form impossible. Sixteen feet is not generous; it is the number below which the form stops working.
§ 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.
Without it, a transit-adjacent lot would carry the same height ceiling as any other, and the two extra feet for roof pitch is the detail that keeps the allowance from being consumed by the roof itself.
§ 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.
Without it, an attached unit would be capped by the ADU height rule rather than by the house it is attached to — which would produce the absurd result of an addition that cannot match the roofline it joins.
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 the provision that makes conversions work. Without it, a garage sitting two feet from the line could not become a dwelling without being moved, which in practice means demolished. Read together with Gov. Code § 66323(a)’s bar on requiring correction of nonconforming zoning conditions, it is the reason an old, badly-sited outbuilding is an opportunity rather than a liability.
Quoted at the numbering current on the retrieval date. A local agency shall not impose any objective development or design standard that is not authorized by this section upon any unit that meets the requirements of any of paragraphs (1) to (4), inclusive, of subdivision (a). — Gov. Code § 66323(a).
Which units qualify for that protection differs by type, so read the type selector alongside this.
A floor is not a permission, and the difference is where people go wrong
The most common misreading of the statutory floor is to treat it as an entitlement to 800 square feet anywhere. It is not. It is a limit on what the agency may require, and the categories it attaches to are narrower than the summary suggests.
The by-right categories are their own list. Gov. Code § 66323(a): “Notwithstanding Sections 66314 to 66322, inclusive, a local agency shall ministerially approve an application for a building permit within a residential or mixed-use zone to create any of the following units” The detached category reads: “One detached, new construction, accessory dwelling unit that does not exceed four-foot side and rear yard setbacks for a lot with a proposed or existing single-family dwelling.” Notice how much work the words detached, new construction and one are doing.
A scheme that is none of those things is being evaluated under a different paragraph with different terms.
And where they apply, nothing may be added. “A local agency shall not impose any objective development or design standard that is not authorized by this section upon any unit that meets the requirements of any of paragraphs (1) to (4), inclusive, of subdivision (a).” That sentence is the reason it is worth knowing exactly which paragraph your scheme falls under: inside one of them, an objective design standard the agency invents is not available to it.
Outside them, the ordinary local standards apply in full and the state floor is only a floor.
What that means for the arithmetic on this page. If your scheme sits inside a by-right category, a coverage rule or a setback that would prevent 800 square feet with four-foot side and rear setbacks is not a ceiling you have to respect — and the calculator, which simply multiplies the numbers you type, will still report it as binding.
That is a limitation of the tool, not a finding about your lot. It computes geometry; it does not apply statutory override.
The practical instruction. Compute the three ceilings first, then take them to the counter with one question: which of these the agency considers applicable to this scheme, given the state floor.
That is a much better question than “how big can I build”, because it is answerable and it puts the disagreement, if there is one, on the table early rather than at plan check.
Seven hundred and fifty square feet is a design dimension, not a fee footnote
Seven hundred and fifty square feet is the most consequential number in an ADU project, and it is the one most often crossed by accident — by a drawing that looked balanced on a page.
Note that the protected floor (800) sits above the fee threshold (750). The statute protects your right to build a unit that will be charged impact fees. Those are two different provisions doing two different jobs, and reading them as one rule is the mistake.California Legislature, Gov. Code §§ 66310–66342. Retrieved 2026-09-05.
What the threshold does. Impact fees may not be imposed on an ADU of 750 square feet of interior livable space or less (or a JADU of 500 or less). Above 750 square feet, impact fees must be proportional to those for the primary dwelling.
School developer fees under Education Code § 17620 are a different statute. Section 66311.5(c)(3) treats an ADU or JADU under 500 square feet of interior livable space as not increasing assessable space by 500 square feet.
What crossing it costs. 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.
We do not publish a figure for what your jurisdiction charges, because we hold no fee schedule and inventing a magnitude would be worse than publishing none.
But the shape is not in doubt: it is a step, not a slope, and the step lands between two units that differ by a couple of square feet.
Which makes it a design instruction. A designer who knows about the threshold hits it deliberately.
A designer who does not draws a comfortable plan that lands at 780 square feet because the room proportions worked, and nobody in the process ever writes down that a five-figure charge was accepted in exchange for thirty square feet.
That is not a design decision badly made; it is a design decision made invisibly, which is worse.
Interior livable space is not gross area. The statute measures the threshold in interior livable space, which is a different quantity from the gross area a builder quotes, from the footprint a coverage rule counts and from the area an assessor records.
Exterior walls, an attached porch and unconditioned storage can all sit on the wrong side of that distinction.
If a scheme is anywhere near the line, ask the agency in writing which measurement it applies and to which drawing — because a plan that is under the threshold on one measure and over it on another will be assessed on theirs.
The other threshold, at 500. 500 square feet is the junior unit’s equivalent, and the school-fee statute has its own treatment at the same number.
Small units therefore sit inside two separate protections rather than one, which is part of why the economics of a genuinely small unit are better than a linear reading of cost per square foot suggests.
A room that cannot make seven feet is not a cheap ADU. It is not an ADU
The zoning ceilings decide how big the building may be. The model code decides whether a given space inside it is habitable at all — and in conversions that is the question that ends schemes.
The rule. 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).” 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).” 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).”
Why it matters more than any zoning number. 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.
Read the sloped-ceiling exception carefully, because it is more generous than people assume and narrower than they hope. It does not permit a low room.
It permits a room in which at least half of the required floor area has seven feet, with nothing below five feet counting toward the required area at all.
In an above-garage space under a pitched roof that is often the difference between a workable plan and an impossible one, and it is decided by the existing roof geometry rather than by anything you can choose.
And the wet rooms get their own number. 6 ft 8 in for bathrooms, toilet rooms and laundry rooms. That four inches of relief is occasionally what makes a basement scheme work — the plumbing goes under the lowest beam and the habitable space goes where the height is.
The honesty caveats, in the text rather than in a footnote. 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.
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.
What to do with that. Measure the actual clear height before anything else in a conversion — floor to the underside of the lowest thing, joist, beam or duct, not to the ceiling finish you can see.
Then confirm the adopted figure with the authority that will inspect, because adoption and amendment are local and we are deliberately not labeling these with an edition we could not verify.
Emergency egress: three dimensions that must be satisfied at the same time
The second habitability constraint, and the one most often failed by a window that looks perfectly adequate. The final column is why this is a size question rather than a joinery question.
The requirement
The figure
The provision
What it is really testing
Net clear opening area
5.7 sq ft (5 sq ft at grade floor)
“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).”
Whether a firefighter in breathing apparatus can get in. It is the dimension that fails, and it fails on windows that comfortably pass the other two — because area is the product of the height and width that remain after the sash and frame, not of the opening you measured on the outside.
Minimum clear opening height
24 inches
“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.”
Whether the opening is tall enough to pass a body through. Note the closing sentence: the dimensions are the result of normal operation. A window that only meets the figure when the sash is removed does not meet the figure.
Minimum clear opening width
20 inches
Same provision
Whether the opening is wide enough. Meeting the minimum height and the minimum width simultaneously produces only 3.3 square feet — well under the area requirement. The three are a joint constraint, not a menu, and that arithmetic is the trap.
Maximum height to the bottom of the clear opening
44 inches above the floor
“Emergency escape and rescue openings shall have the bottom of the clear opening not greater than 44 inches (1118 mm) above the floor.”
Whether an occupant can reach and climb through it. 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.
Where it applies
Basements, habitable attics and every sleeping room
“Basements, habitable attics and every sleeping room shall have not less than one operable emergency escape and rescue opening”
Whether the space is being used as a bedroom, which is the question that decides whether the whole requirement engages. A basement conversion with an open plan and no designated sleeping room is a different proposition from the same space with a bedroom in it — and the second is what most ADU schemes need in order to let.
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.
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.
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.
Smaller is not proportionally cheaper, and the survey spread shows why
Halving the floor area does not halve the cost, because the expensive parts of an ADU are not the parts that scale with area. This is the reason a size decision is a budget decision only loosely.
The spread is the finding, not the median. 37% of that sample reported under $100,000 and 71% under $200,000 — against a median of $150,000 and a median of $250 per square foot.Chapple, Ganetsos and Lopez, Implementing the Backyard Revolution: Perspectives of California’s ADU Owners, UC Berkeley Center for Community Innovation, 22 April 2021
What the survey measured. Chapple, Ganetsos and Lopez, Implementing the Backyard Revolution: Perspectives of California’s ADU Owners, UC Berkeley Center for Community Innovation, 22 April 2021 reported a median all-in cost of $150,000 and a median of $250 per square foot among California owners who had actually built.
That is a real measurement of real projects — and it was taken in 2021.
What the spread tells you about size. If cost tracked area closely, the distribution would be tight around the median, because ADUs do not vary that much in size. It is not tight.
Which means area is one input among several, and the others — site work, the utility connection, the foundation, whether an existing shell is being reused — are doing at least as much work.
A per-square-foot rate multiplied by your envelope is therefore a weak estimate, and it is weakest at the small end where the fixed costs dominate.
The indexed figures, with the publisher’s own warning. Indexing that 2021 statewide median forward by 1.54 gives $230,351. That arithmetic is ours, on DGS California Construction Cost Index, ENR Building Cost Index average of Los Angeles and San Francisco.
HyreADU arithmetic on published indices applied to Chapple, Ganetsos and Lopez 2021 medians. Not a 2026 owner survey and not a bid.
And the index publisher states plainly: “The ENR BCI reports cost trends for specific construction trade labor and materials in the California market and does not reflect current market bidding environment.” An index adjusts a past number for input costs. It does not turn a 2021 survey into a 2026 quote.
Which is why the envelope is not the budget. We hold no dataset of ADU bids, quotes or contracts. Every dollar figure on this site is a named third party’s published figure, arithmetic we performed on one and have labeled as ours, or a number you typed in.
The useful sequence is to establish the envelope here, decide the fee-cliff question deliberately, and then take a defined size to the cost calculator as line items rather than as a rate multiplied by an area.
How to get numbers worth putting in the form
The arithmetic is trivial. Getting inputs that mean something is the entire job, and three of these five steps involve a tape measure rather than a website.
1
Get the lot area from the record, not from memory
The assessor’s figure, or a survey if one exists. “About a quarter acre” is not an input. Where a survey and the assessor disagree, the survey is the document a plan checker will accept.
2
Ask planning what counts as coverage before you total it
House, garage, patio, shed, covered porch, and sometimes a driveway. The list is local and it is not obvious. Totalling the wrong set of structures produces a confident wrong answer, and the tool cannot tell the difference.
3
Tape the pad inside the setbacks, on the ground
From the property line, not from the fence — they are often not the same line. Rear and side setbacks usually decide whether a detached unit fits at all, and the rectangle that survives them is almost always smaller than the yard looks in a photograph.
4
Copy the zoning maximum from the current ordinance, as text
Not from a summary, a blog or a handout of unknown vintage.
If you cannot find it, leave the field at zero — the result will tell you that ceiling is unknown, which is a more honest output than a guess.
And check the numbering the ordinance cites; a local ADU chapter still pointing at § 65852.2 predates March 2024.
5
Measure clear ceiling height before you consider any conversion
Floor to the underside of the lowest obstruction. This single measurement decides whether a basement or above-garage scheme is possible, and it costs nothing to take. Take it before you pay for anything at all.
Footprint, floor area and interior livable space are three different quantities
Almost every confusion on this page reduces to using one word for three measurements that different rules apply to. Separating them once makes the rest of the arithmetic behave.
What each one measures
Footprint is the area the building occupies on the ground. It is what a lot coverage rule counts, and it is what the pad inside the setbacks has to hold. A second story adds nothing to it.
Floor area is the total across all stories. It is what a zoning maximum usually caps, and it is why a two-story unit can have more floor area than the pad — where height and stories are permitted, which is a separate question for the counter.
Interior livable space is the statutory measure the fee thresholds turn on. It is neither of the above and it is smaller than gross area. If a scheme is near a threshold, the measurement definition is worth a written question rather than an assumption.
Where the confusion costs money
A builder quotes on gross area, a coverage rule counts footprint, and a fee threshold measures interior livable space. Three numbers, one word, and the drift between them is exactly the size of the margin people leave against the 750 square foot line.
The calculator on this page compares like with like only if you feed it like with like. Remaining coverage is a footprint quantity. A zoning maximum is usually a floor-area quantity.
Comparing them directly is valid for a single-story scheme and misleading for a two-story one, and the tool has no way to know which you are drawing.
The honest workaround is to run it as a single-story footprint question first, establish whether the pad binds, and only then ask the counter what height and stories are available — because a second story changes which ceiling is the smallest one.
This is the reason the tool asks for pad width and depth separately rather than an area: a long thin rectangle and a square of the same area hold very different buildings.
Why getting the envelope right early is worth more than it looks
46,944 of 114,651 permitted ADU units in that cohort have no completion row matched to them. Reported APR rows, not a physical census of construction.California Department of Housing and Community Development, Housing Element Annual Progress Report, Table A2. HyreADU calculation on Table A2.
The state’s own reporting. Across 2018-2025, California Department of Housing and Community Development, Housing Element Annual Progress Report, Table A2 recorded 178,495 ADU permits and 102,356 completions. Following the 2018–2023 permit cohort forward, 41% of permit units have no matching completion.
Read the caveat with the number: 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.
What that has to do with an envelope. Our reading is that the projects which stop, stop after money has been committed — and a size decision made late is one of the more common reasons money gets committed to the wrong thing.
A scheme redrawn because the pad was measured to the fence rather than the property line, or because the basement could not make seven feet, has paid twice for one design.
The cheap version of this whole page. Four measurements and two phone calls: lot area, existing coverage, the pad inside the setbacks, clear ceiling height in any space you might convert; then planning for what counts as coverage and what maximum applies, and the utilities for capacity.
None of it costs anything and all of it is upstream of the first invoice.
What to ask, and what a good answer sounds like
Six questions for the counter and one for yourself. Each has a shape of answer that is useful and a shape that is not, and knowing the difference is most of the value.
Ask exactly what counts toward lot coverage here
A good answer is a list: house, garage, covered patio, shed, and whether uncovered paving counts. A bad answer is a percentage with no list attached, because the percentage is useless until you know what it is a percentage of.
Ask which paragraph of the state article the agency considers your scheme to fall under
The by-right categories at Gov. Code § 66323(a) carry “A local agency shall not impose any objective development or design standard that is not authorized by this section upon any unit that meets the requirements of any of paragraphs (1) to (4), inclusive, of subdivision (a).” with them.
Knowing which paragraph applies tells you whether a local standard is a real ceiling or one the state has already displaced.
Ask, in writing, how interior livable space is measured for the fee threshold
The threshold sits at 750 square feet. A good answer names the measurement convention and the drawing it is taken from. If the scheme is within about fifty square feet of the line, this question is worth more than any other on the list.
Ask whether height and a second story are available on this parcel
The pad is a footprint constraint; a second story converts it into floor area. A good answer gives a height in feet and a story count, and says whether the state height figures apply to your type. It changes which of the three ceilings binds.
Measure the clear ceiling height yourself before you ask anyone anything
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.
Floor to the underside of the lowest joist, beam or duct. If it is short of seven feet across the space that would be habitable, the conversion question is closed and the money is better spent elsewhere.
Check the egress opening against all three dimensions at once
5.7 square feet net clear, 24 inches high, 20 inches wide, and not more than 44 inches to the bottom of the clear opening. Meeting the minimum height and width alone gives 3.3 square feet, which is not enough. Confirm the adopted figures locally.
Do not treat the tool’s smallest number as a permitted size
It is arithmetic on inputs you supplied. If a setback is wrong or a coverage definition is different locally, the output is confidently wrong in exactly the same format as a correct one.
Do not design to the maximum by reflex
The largest permitted envelope is not the best project. It crosses the fee threshold, it costs more to build, and the survey spread above suggests area is a weaker driver of cost than the fixed items are.
Build the size that suits the use, then check it against the ceilings — not the other way round.
What this calculator cannot do, specifically
Real boundaries rather than a disclaimer, because knowing where the arithmetic stops is what stops you over-trusting the output.
It cannot read your ordinance. Every ceiling in it is a number you typed. Type the wrong setback and it will report a confident wrong pad, in the same typeface as a right one. It has no zoning map, no parcel record and no way to detect that an input is implausible for your jurisdiction.
It cannot apply the statutory override. Where the state floor displaces a local standard, the arithmetic here will still report that local standard as binding, because it multiplies what it is given. Reading the floor sections above alongside the output is the manual step that no calculator on a national page can do for you.
It cannot survey. Property lines are not fences and fences are not property lines. Where the lines are genuinely the whole question — a tight side yard, a corner lot, an old subdivision — a surveyor is the cheaper answer, and a GIS parcel map is a starting point rather than a construction document.
It cannot see a second story, a slope or an irregular lot. It multiplies a width by a depth. A wedge-shaped rear yard, a slope that turns a foundation into an engineered element, or a scheme with accommodation over a garage all break that assumption in ways the output will not flag.
It cannot tell you what the space will cost. We hold no dataset of ADU bids, quotes or contracts.
Every dollar figure on this site is a named third party’s published figure, arithmetic we performed on one and have labeled as ours, or a number you typed in.
And per the spread above, a per-square-foot rate is a weak instrument at ADU scale in any case.
It cannot handle a conversion properly. New coverage and a new pad frequently do not apply — Gov. Code § 66314(d)(7) removes the setback question for a structure converted in place. The binding constraints there are habitability, not geometry, and they are in the ceiling-height and egress sections above rather than in the form.
And it will not tell you that you may build. Binding arithmetic on numbers you typed is not a permit and not a determination of eligibility. 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.
How this calculator works
Remaining coverage, when you have entered both a lot and a coverage percent:
remaining coverage = lot × (cover% / 100) − existing coverage
pad = pad width × pad depth
binding = min(remaining coverage, pad, zoning max)
(only among the numbers that actually exist)
A field at 0 is treated as unknown, not as zero square feet of permission. If coverage percent is 0, coverage is dropped from the minimum. If zoning max is 0, it is dropped and the result says so, because that cap is often the binding number.
If remaining coverage is negative, the result says existing coverage already exceeds the maximum you entered. That is a data problem or a “what counts” problem, not a green light to ignore the rule.
What each input means
Inputs on this tool, in the order they appear on the form.
Input
What it is actually asking
Lot area
The parcel, in square feet. Assessor maps and surveys disagree sometimes. For a tight envelope, use the survey.
Maximum lot coverage %
From the zoning ordinance. What counts (house, garage, driveway, patio, eaves) is a local definition. 0 = unknown.
Existing coverage
What is already on the lot under that definition. Over-counting is safer than under-counting.
Pad width × depth
The rectangle that remains after applying rear and side setbacks, and after leaving any required separation from the house. Not the whole backyard.
Zoning max
The current maximum ADU floor area on this parcel. Copy it. Do not type a number you remember from a podcast about California.
Worked examples
Including one where the naive answer misleads, which is the example most calculators leave out.
Coverage binds before the pad does
Lot 6,000 sq ft, 40% coverage, 1,800 sq ft existing, pad 30 × 40 ft (1,200), zoning max 800. Remaining coverage = 2,400 − 1,800 = 600 sq ft. Pad 1,200. Zoning 800.
Binding: coverage at 600. This is the engine test. A cottage drawing of 800 sq ft would fail coverage even though the pad and the cap look fine.
Zoning max missing
Same lot and pad, coverage unknown (0%), zoning max left at 0. The tool can only report the pad: 1,200 sq ft. It will say the zoning max was not entered.
That is the honest result. The naive result would be “you can build 1,200 sq ft,” which is exactly the sentence this page exists not to print.
The one where a backyard photo misleads
A wide-looking yard on a 50 × 100 lot with 20 ft rear and 5 ft side setbacks. The photo is the lawn.
The pad is 40 × 20 ft if the house already sits on a 30 ft front setback and a 50 ft depth of house+yard math that you have not done. Run the numbers.
The naive answer (the lawn is the pad) is how detached gets chosen on a lot that only holds attached.
What changes the result
Existing coverage is the quiet killer. A patio that counts, or a garage you planned to keep, can erase the remaining 400 sq ft you thought you had.
A second story is not in this arithmetic. If height is permitted, floor area can exceed the pad. If height is not, the pad is the floor area. Ask planning about stories before you treat pad and floor area as the same number.
Conversions of existing space often skip the pad test and sometimes skip coverage. Do not force a conversion through this detached-pad model. Ask which envelope applies.
Local considerations
Coverage definitions, setback tables and ADU size caps are local, and they have moved.
Some states cap how small a city may force an ADU to be, or how large a setback it may demand. Those statutes are state-specific.
We will not paste California’s numbers onto a national form. Type the current local figures, or leave them at 0.
When not to use this
Do not use this as a survey, a site plan, or a permit. Do not give the binding number to a designer as “the city said.” The city has not said it. You typed it.
Do not use it for a conversion unless planning told you coverage and a new pad apply. Use the feasibility checker first.
HyreADU does not survey lots and does not establish eligibility.
Related on this site
Feasibility factorsProperty size, setbacks, lot coverage, existing structures.
The six formsDetached is the form that needs this pad.
Questions this calculator answers
How big can my ADU be?
The minimum of: remaining lot coverage, the pad after setbacks, and the maximum floor area in the ordinance, plus any other local limits (height, stories, matching the house). This tool compares the three numbers you can type. It does not know the others.
What is lot coverage?
The share of the lot that structures are allowed to cover. The house, garage, patio and shed usually count before the ADU does. A 6,000 sq ft lot at 40% coverage with 1,800 sq ft already built has 600 sq ft left under that rule, before setbacks and a size cap are applied.
What are setbacks?
How close a structure may sit to each property line. Rear and side setbacks usually decide where a detached ADU can physically go. Measure the remaining rectangle; do not assume the backyard photo is the pad.
Why is the zoning max a field I type?
Because it is local, it has moved in several states, and a national table of ADU size caps would be wrong on arrival. Copy the current number from the ordinance or from planning. 0 means “I have not looked it up,” which the result will say.
Does floor area equal the pad?
No. A two-story ADU can have more floor area than the pad, if height and stories are permitted. This tool does not assume a second story. If you are planning one, the pad is the footprint, not the floor area, and height becomes the next question for planning.
What if remaining coverage is negative?
Existing structures already exceed the coverage maximum you entered. Either the percent is wrong, something on the lot does not count as coverage locally, or there is no room under that rule. Ask planning what counts.
Can I ignore coverage if the ordinance has an ADU exemption?
Some jurisdictions exempt ADUs from coverage or from some setbacks. That is a local exception, not a national one. If planning tells you coverage does not apply, leave cover % at 0 and the tool will ignore that ceiling.
Is this a survey?
No. Tape the lot and the setbacks, or hire a surveyor if the lines are the whole project. A GIS parcel map is a start, not a construction set.
Does a conversion use this tool?
The pad and new coverage often do not apply to a conversion of existing space. The zoning max still might. Conversion is a different envelope. Confirm with planning, and use HyreGarage for conversion cost lines.
Does this mean I can build that many square feet?
No. Binding arithmetic on numbers you typed is not a permit. HyreADU does not survey lots and does not establish eligibility.
Sources and methodology
Figures dated 26 August 2026. Last reviewed .
Adopted state and local building codes (International Code Council, retrieved 2026-08-26. Height, stories, egress and fire separation sit in the adopted code. Lot coverage and ADU size caps sit in the zoning ordinance. This tool does not look either up for your parcel.)