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ADU cost calculator: a budget you can audit

Eight lines, every figure yours. There is no universal ADU price, and a national $/sf would hide the two lines that most often blow a budget: site work and utility connection.

Your lines not a national ADU average The cost page on this site already refuses to publish an average ADU price. This tool is that refusal made arithmetic: you type design, fees, site, utilities and a building rate from a bid, then hold a contingency yourself.

The ADU cost calculator adds up the budget lines you type: design, permits and fees, site work, utilities, the building and a contingency you hold. Every dollar field starts empty.

Type figures from your own bids or from your city’s fee schedule, and the total is your arithmetic, never a quote. HyreADU does not build ADUs and publishes no average ADU price.

Eight lines, then a total

Empty lines stay $0. The building rate is yours. Nothing is emailed.

Habitable floor area you are actually drawing, not the lot.

0 until you have a real number. We will not invent one.

Access, slope, soil, drainage, demolition, the pad.

Water, sewer or septic, gas, electrical capacity. Often the surprise.

A second bathroom, or a kitchen that was not in the building rate.

— Budget (your figures)
— Building line
— Design + fees + site + utilities
— Contingency
— What this form usually hides

What this assumed —

A budget you typed, never a quote. No national ADU $/sf is shipped. HyreADU does not build ADUs.

There are four different “ADU costs”, and they are not the same quantity

Almost every disagreement about what an ADU costs is two people quoting two different measurements at each other. One is a number written on a fee form.

One is what owners said they spent, five years ago. One is our own arithmetic on the second. One is a bid. Only the last is a price, and only you can get it.

Start by refusing the question. “How much does an ADU cost” has no answer at the national level, and the honest response is not a smaller number with more caveats — it is a different question.

The useful one is: which of the four quantities below is the figure in front of me, who produced it, for what purpose, and what would make it wrong in my direction?

Declared permit valuation. When an applicant pulls a building permit, a value goes on the form so the city can compute its fee. Declared permit valuation is what an applicant writes on a form so a city can compute a fee.

It is not a construction cost, it is not a bid, and it is not what the job was sold for. Where a fee scales with the number written down, the number written down has an obvious direction of error.

An owner survey. Chapple, Ganetsos and Lopez, Implementing the Backyard Revolution: Perspectives of California’s ADU Owners, UC Berkeley Center for Community Innovation, 22 April 2021 asked people who had actually built one what they spent.

It is a real measurement of real projects, it is several years old, and it is not a price for your lot, so we do not repeat its figures as a guide to your budget.

Our indexation of that survey. We carried the survey medians forward with a published construction cost index, which is arithmetic we performed and label as ours every time it appears.

Indexing tells you what the same building would cost at today’s trade labor and material prices. It does not tell you what anyone would quote you.

A bid on your drawings. The only figure that is a price.

It is specific to your parcel, your set of drawings, your city’s fee schedule and the state of one builder’s order book.

Everything above it is context for reading it, never a substitute for having it. 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 two lines that break budgets, and why they do not scale with floor area

The building rate is the line everyone negotiates. It is rarely the line that ruins the project. These two are, and both are structurally bad at being expressed as a cost per square foot.

Site work and foundation

Site work is everything that has to be true before a building can go up: equipment access, slope, soil, drainage, tree protection, demolition, and the pad itself. None of it is proportional to the floor area of the unit you are building.

The mechanism is simple once stated. A retaining wall is priced by the height of the ground it holds back, not by the size of the building behind it.

An access route is priced by what has to be removed to get a machine to the back of the lot. Doubling the unit from 400 to 800 square feet does not double either.

This is exactly why a single national dollar-per-square-foot figure is not just imprecise but structurally wrong: it applies a per-area rate to costs that are not per-area quantities.

What to do about it: keep site work as its own line, at zero, until somebody who has stood on the lot prices it. A zero that is visibly unpriced is a more useful budget than a plausible allowance that quietly becomes the number everyone plans around.

The utility connection

This is the line that most often turns a viable project into an abandoned one, and in California it has a specific legal shape that almost nobody explains.

Connection and capacity charges are NOT impact fees. Under § 66311.5(b), an ADU is not a new residential use for connection-fee purposes unless it is built together with a new single-family dwelling — but the utility can still bill a connection or capacity charge, and that bill is outside the impact-fee prohibition entirely.

Read that twice, because it is the most expensive sentence on this page. The statutory relief people have heard about is relief from impact fees.

Connection and capacity charges are a different instrument, billed by a different body, and they sit outside the prohibition entirely.

A homeowner who has read that “ADUs under a certain size pay no fees” and budgeted accordingly has budgeted for the wrong category of charge.

And we deliberately publish no dollar figure for it. We could not retrieve a municipal or utility fee schedule we were willing to stand behind, so we publish the statutory caps and prohibitions and tell you to get your own two utilities’ figures in writing.

An invented magnitude here would be worse than none, because it would be believed.

Why both are absent from cheap bids

A bid competing on headline price has an obvious incentive: move the lines nobody can verify at signing into allowances, and let them settle later as change orders when you have no bargaining power left.

The test is not whether the number is low. It is whether the number is a price or a placeholder. Ask directly: is the site work priced or allowanced? Is the utility connection priced or allowanced?

Who carries the difference if the trench is longer than assumed? A builder who answers those three cleanly is telling you something real about how they run jobs.

The 750 square foot cliff is a design decision with a fee attached

Most size decisions are about how you want to live. This one is about money, and it is discontinuous — which means it does not show up anywhere on a cost-per-square-foot curve.

The rule. 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.

That sits at Gov. Code § 66311.5, and the fee provisions were renumbered recently: The ADU fee rules that used to sit in former § 65852.2, and then at § 66324, were renumbered to § 66311.5 by SB 543 (Stats. 2025, ch. 520, effective 1 January 2026).

Why it matters more than its size suggests. 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.

A cliff is not a slope. Every other cost line on this page moves smoothly with area; this one steps.

Which means the marginal cost of the 751st square foot is not the building rate — it is the building rate plus the whole fee package that square foot just switched on.

There is a second, lower threshold. 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.

So the 500 square foot line and the 750 square foot line are different thresholds in different statutes doing different jobs, and a junior unit has its own at 500.

Anyone who tells you there is “a fee cutoff” without saying which one is compressing three rules into one.

How to use it in a design conversation. Do not ask your designer to hit a number. Ask them what the interior livable area is on the current scheme, and then ask your planning counter what fees attach above and below the threshold on this parcel.

If the answer is a large one, you now have a costed reason to consider a slightly smaller unit — and if the answer is small, you have permission to stop optimising and design the home you want.

The conclusion that says do nothing. If your scheme is already comfortably under the threshold, this whole section is not your problem, and shrinking further to chase fees you were never going to be charged is a real loss of living space in exchange for nothing. The cliff only bites near the cliff.

What a good bid says — and the questions that get you there

Reading the bid is the highest-value section on the page. None of it requires you to know anything about construction; it only requires you to notice which questions get a straight answer.

  • Which of these lines is priced, and which is an allowance?

    Ask it line by line: design, permits and fees, site work and foundation, utility connection, the building itself, wet rooms. An allowance is a placeholder that will be reconciled against actual cost later.

    That is legitimate — but you need to know which lines are placeholders before you compare two bids, because a bid made of allowances will always look cheaper than a bid made of prices.

  • What is the interior livable floor area on this scheme, exactly?

    Not the gross footprint, not the roof area. Interior livable area is the measurement the 750 square foot fee threshold uses, and it is also the measurement the size limits in the statute use. A scheme described as “about 800 square feet” is not a scheme you can test against a threshold.

  • Have you asked both utilities what they will charge to serve a second dwelling here?

    Both, separately, in writing — water and sewer are frequently different bodies from the electricity supplier, and each has its own connection and capacity schedule. This is the question we refuse to answer with a national figure, because the honest answer is only available from them.

  • Which fees on your permit line are impact fees, and which are connection or capacity charges?

    They behave differently under the statute. The prohibition reaches one and not the other. If your builder cannot separate them, they do not know which of the numbers in their permits line are actually capped and which are not.

  • Where does your building rate come from?

    The right answer is “from our own recent jobs of this type in this city”. A rate lifted from a published average — including ours — is a rate nobody has stood behind. Ask which recent projects, of what type, and how the final cost compared to the bid.

  • Who carries the risk on unknown ground conditions?

    Somebody does. Either the price includes an allowance you can see and a mechanism for reconciling it, or the risk sits with you as a change order. Both are workable. Not knowing which one you have signed is not.

  • What is your payment schedule tied to?

    Progress that can be observed — a poured foundation, a passed inspection, a framed and dried-in shell — rather than dates on a calendar or the builder’s own cash needs.

    If a finance draw schedule also exists, ask whether the two schedules line up, because when they do not the gap is funded out of your pocket.

  • What would make you walk away from this project?

    An unusual question and a revealing one. A builder with real experience of ADUs has a list: a sewer they cannot reach, access they cannot get a machine through, a slope that turns into engineering. Someone with no list has either not built many or is not telling you about the ones that went wrong.

Why costing honestly early is the whole point

California publishes both columns of its own reporting, and the gap between them is the strongest argument we know for spending a small amount of money finding out the truth before spending a large amount assuming it.

The figure. Following each permit year forward, 41% of ADU permit units issued between 2018 and 2023 — 46,944 of 114,651 — have no matching completion in the state’s file. That is a HyreADU calculation on the published dataset, and the method is on the research page.

Read the caveat before you use it. 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 holds; the exact rate is a reporting artefact as much as a construction one.

Why it belongs on a cost page. Everyone in that unmatched group had already paid for drawings, paid fees, and obtained an approval.

Whatever stopped them stopped them after the money started going out and before a building existed.

Money is not the only thing that stops a project, but it is the one this page can help with.

What early costing actually costs. A conversation at the planning counter is free. A written answer from two utilities is free.

A site visit from a builder who has done this in your city is usually free or nearly so. A soils or survey report is not free but is small against a construction budget.

All of those can be had before you commission a full drawing set, and any one of them can tell you the project does not work — which is the cheapest possible way to find out.

The sequence we would follow. Establish that the use is permissible on this parcel. Get the fee schedule. Get both utility answers in writing. Get someone to look at access and ground.

Only then commission the drawings that a bid can be priced against. Every step in that order is cheaper than the one after it, and each one can end the process before the expensive step.

Two conclusions this calculator is allowed to reach

A tool that can only ever say “here is your number” is a sales instrument. These are the two outputs worth taking seriously.

Leave the line at zero

If you do not have a building rate from a person who has priced this form of work in your city, leave it at zero and add up what you do have.

A total that is visibly incomplete is a working document. A total built on a rate you found in a roundup is a fiction with a decimal point.

The same applies to utilities and site work. Zeros in those lines are not optimism; they are honesty about what you have not found out yet, and they keep the missing questions visible every time you look at the sheet.

Keep what you have

If the honest budget — with real site and utility numbers in it — does not work against what the space is worth to you, the correct answer is to stop. Not to shrink the unit until the arithmetic clears, and not to move the contingency into hope.

A garage that stays a garage costs nothing more. That is a legitimate outcome of running these numbers, it is the outcome for a meaningful share of people who run them, and it is a conclusion we can print because we do not build, sell or finance anything on this site.

What this calculator cannot do

Specific blind spots, not a disclaimer. Each of these is a real way the total in front of you can be wrong.

It cannot see the ground. Soil bearing, groundwater, fill from an old structure, rock, a slope that needs engineering — all of it lands in one line you typed a guess into. This is the single largest source of error in the output and it is entirely invisible to the form.

It cannot see access. Whether a machine, a concrete truck, a crane or a modular module can physically reach the build location changes the method of construction, not just its price. A prefab unit that cannot be craned in is not a prefab unit.

It does not know your fee schedule. It multiplies nothing by location. We ship no state or city multiplier because we hold no sourced series to build one from, and a fake localiser is worse than none — it produces a number that looks specific to you and is not.

It does not price the ADU type. Choosing detached or conversion changes the warning under the total, not the arithmetic. Applying a “conversion discount” would mean publishing a national index we do not have.

The indexed type medians in the indexation section above are the closest honest thing, and they are our arithmetic on a survey, offered as magnitude rather than as a multiplier.

It cannot tell you whether you may build. Cost is downstream of permissibility. A perfect budget for a unit the jurisdiction will not permit is a perfectly wrong document. The feasibility checker produces the questions for that conversation, and even it will not say yes.

It has no view on time. A budget is a snapshot; a build is a duration. Materials, labor availability and fee schedules all move over the life of a project, and a longer program is also a longer exposure to all three. The timeline planner is the tool for that side of it.

And the standing disclosure. 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.

How this calculator works

The calculator adds the lines you type, then applies the contingency percentage on top:

building  = floor area × your $/sf
subtotal  = design + permits/fees + site/foundation + utilities + building + extra wet-room
total     = subtotal × (1 + contingency)

ADU form is a label. It does not multiply anything. If we applied a “conversion discount” or a “detached premium” we would be publishing a national index we do not have. The cost page on this site already says there is no universal ADU price; this tool is that sentence turned into arithmetic.

Garage conversion is a special case. The envelope exists, so the building rate on this form should not be a new-house $/sf.

The lines that actually exist on a conversion (insulation, floor, electrical, plumbing, HVAC, egress, parking lost) are canonical on HyreGarage’s conversion cost calculator.

Use that tool for those lines, then bring fees and utilities back here if you are treating the result as an ADU.

What each input means

Inputs on this tool, in the order they appear on the form.
InputWhat it is actually asking
ADU form Detached, attached, conversion, basement, above-garage, prefab. Changes the warning under the total, not the dollars.
Floor area The habitable square footage you are drawing. Not the lot, not the garage footprint unless that is the unit.
Building rate Installed construction $/sf from a person who has priced this form in your city. 0 is the honest default.
Design / engineering / survey Architecture, structural, civil or survey, energy-compliance documents where required. Often a percentage of construction; type the bid.
Permits and fees Building permit, planning review, and any impact, school, park or utility-capacity fee that attaches to a new dwelling. Ask planning for the schedule.
Site and foundation The least visible line. Access for equipment, slope, soil, drainage, tree protection, demolition, the pad. Does not scale smoothly with floor area.
Utilities Water, sewer or septic, gas, electrical service: capacity, trenching, meters, connection fees. The other line that does not scale smoothly with floor area.
Extra wet-room A second bathroom, or a kitchen that was not inside the building rate. Wet rooms carry disproportionate cost per square foot.
Contingency A percentage you hold. Ten percent is a placeholder. Unknown soils and unknown utilities want more.

Worked examples

Including one where the naive answer misleads, which is the example most calculators leave out.

Two bids on the same drawings

Type each bid into the form, one at a time, line by line. If the totals differ, look at which line moved. A gap in the building line is a pricing difference. A gap in site work or utilities usually means one bid priced the ground and the other carried an allowance.

Utilities still unknown

Leave the utilities line at 0 if no one has priced the connection yet. The total will look smaller. That is not a cheaper project.

It is a project with a hole in it. Keep the line visible until the water and sewer providers have told you what serving a second dwelling on this lot needs.

Why one rate per square foot misleads

A garage conversion and a detached cottage can carry the same advertised total and fail for different reasons. The conversion still needs legal egress, a habitable floor and an answer on parking.

The cottage still needs a foundation, a roof and a sewer run. Type the lines. If the form is a garage conversion, use the HyreGarage tool for the conversion lines.

What changes the result

The building rate moves the total linearly with area. Site work and utilities often do not. Doubling the unit from 400 to 800 sq ft does not double a sewer tap fee, and it does not double a retaining wall. That is why those two are separate lines rather than baked into $/sf.

Contingency is a multiplier on everything you have already typed. It is not a substitute for a missing utility number. Ten percent of a subtotal that omitted the septic upgrade is not 10% of the project.

Prefabricated and modular invoices look complete until crane access, the foundation, and the utility connection are added. The form already warns about that when you pick prefab. Type those lines anyway.

Local considerations

Location is the single largest multiplier, and it is not on this form as a state dropdown that changes the price. A state labor index without a sourced series would be a fake localiser. Your planning department’s fee schedule, your utility’s capacity charge, and the bids you collect are the local data.

Some states have legislated on ADUs (California’s Department of Housing and Community Development publishes an ADU handbook for that state). That is California law, not yours, unless you are in California.

Neighboring cities in the same metro can still attach different impact fees to a new dwelling. Confirm the current fee schedule for this parcel before you treat the permits line as settled.

When not to use this

Do not use this as a quote, a loan application figure, or proof that the project is affordable. It cannot see soil, access, a crane path, a design-review board, or a utility that will not serve a second meter.

Do not use it to decide whether you may build. That is the feasibility checker, and even that tool will not say yes. Eligibility is a determination about a specific parcel that only the local authority can make.

Do not use a building rate you found in a blog roundup of “average ADU cost.” If you cannot name who priced it, on what drawings, in which city, leave the rate at 0.

HyreADU does not perform ADU work. Matching is still being built. The inquiry form is not a dispatch line.

Related on this site

Questions this calculator answers

How much does an ADU cost?
Whatever the lines add to, in your jurisdiction, on your lot. We do not publish a national average because two identical buildings in two cities can be a long way apart. Size, wet rooms, construction method, site work, utilities, fees and location account for most of the variance. The useful exercise is getting those lines itemized, which is what this tool does.
Why will you not give me a $/sq ft?
Because a single rate pretends site work and utility connection scale smoothly with floor area. They often do not. A short sewer run and a septic upgrade are not the same project. Type a building rate from a designer or a bid; keep site and utilities as their own lines.
Does the ADU type change the price in this tool?
No. Type only changes the disclaimer. A conversion reuses a shell; a detached unit does not; a prefab invoice is not the project. If we multiplied detached by 1.0 and conversion by 0.6 we would be inventing a national index we do not have.
Where should I get the building $/sf?
From a designer who has permitted ADUs in your city, or from a bid on your drawings. Until you have that number, leave it at 0 and add up the lines you do have. An empty building line is more honest than a made-up rate.
What is usually left out of an ADU budget?
Utility connection (trenching, meters, impact and capacity fees) and site work (access, slope, soil, drainage, tree protection, demolition). The cost page on this site already names both. Ask for them to be itemized rather than carried as an allowance.
Is a garage conversion cheaper?
The envelope often is, because it exists. Insulation, floor, egress, electrical, plumbing and the parking you give up are still real lines. The canonical conversion-cost tool is on HyreGarage. This page is the ADU budget around that conversion, including fees and utilities the garage tool does not pretend to know.
Should I hold the contingency inside the contract?
Hold it yourself, outside the contract, unless you like funding other people’s optimism. Ten percent is a planning placeholder on this form, not a rule. Complex sites and unknown utilities want more.
Do permits include impact fees?
Only if you put them in that line. Some cities fold school, park and utility-capacity fees into a “permit” number; some do not. Ask planning for the fee schedule that applies to a new dwelling unit on this parcel, then type it.
Is this a quote?
No. It cannot see soil, a sewer that is further than it looks, a crane that will not fit, or a design-review board. Minus something to plus a lot, lopsided because unseen conditions add money. Use it to tell whether a bid has priced the same lines you have.
Does HyreADU build ADUs?
No. HyreADU is an information and matching layer. Matching is still being built. The form is an inquiry, not a dispatch line.

Sources and methodology

Figures dated 26 August 2026. Last reviewed .

  • Accessory dwelling unit research (U.S. Department of Housing and Urban Development, HUD User, retrieved 2026-08-26. Cited for ADUs as a housing form, not as a national construction price.)
  • Adopted state and local building codes (International Code Council, retrieved 2026-08-26. An ADU is a dwelling. The adopted residential code is the construction rule; this tool does not price it.)
  • Hiring a contractor (Federal Trade Commission, retrieved 2026-08-26. Get itemized estimates in writing. Allowances are placeholders.)
  • Garage conversion cost calculator (HyreGarage, retrieved 2026-08-26. Canonical home for “convert this garage” as insulation, floor, electrical, plumbing, HVAC, egress, permit and parking lines.)

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