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Step 04 · Estimate

What an ADU costs, and what decides it

We do not publish a national ADU price, because one would be wrong for your project by a margin large enough to matter. What we publish is the structure of the cost and the evidence we actually hold.

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The short answer

ADU cost is driven by size, type, site conditions, foundation, utility connections, finish level and your jurisdiction’s fees. There is no single national figure that is useful at the individual-project level. The honest version is a range built from your own site’s line items, which is what our calculator does.

What moves it

The line items that move the number

Roughly in order of how much they swing a project:

  1. 1Foundation

    Pouring new versus reusing an existing slab. The single largest category difference between ADU types.

  2. 2Utility connections

    Water, sewer, gas and power to a rear-yard structure. Distance and whether a trench crosses hardscape both matter.

  3. 3Electrical service

    Whether the existing panel can carry a second dwelling, or whether the service has to be upgraded.

  4. 4Sewer or septic

    A septic system has a capacity, and a second dwelling may exceed it. This is a county health question, not a building one.

  5. 5Size and finish level

    Square footage sets the shell; kitchen, bath and finish specification set the rest.

  6. 6Fees

    The one part of the cost that is genuinely public and lookup-able before you start. See below.

01

The 750 square foot fee cliff

The 750-square-foot threshold is the clearest, best-sourced cost lever in California ADU law, and it is a cliff rather than a slope.

Under Gov. Code § 66311.5, impact fees may not be imposed on an ADU of 750 square feet of interior livable space or less. Above 750, impact fees must be proportional to those for the primary dwelling.

A unit at 749 square feet and one at 751 can differ by five figures in park, traffic and school charges before anyone lifts a hammer. If you are designing near that line, it is worth knowing exactly which side of it you are on.

Connection and capacity charges are a separate matter and are not impact fees — the utility can still bill them.

02

What we will not do

We will not take a national average and multiply it by a regional index to produce a price for your city. That technique is what most ADU cost pages are, and it produces a number with the appearance of precision and none of the substance.

Where we publish construction figures, they come from permit valuations that builders themselves filed with California jurisdictions — a real, dated, sourced dataset with known limits, which the research page explains.

A permit valuation is not a bid. It is what the applicant told the city the work was worth, and it is systematically not the same thing as what the job contracted for.

03

How to get a number you can act on

Three steps, in order. First, look up your jurisdiction’s published fee schedule — it is public and it is the most knowable part of the cost.

Second, price the line items against your actual site using the calculator, which asks you for your figures rather than supplying ours.

Third, get bids on the same set of drawings. Bids on different drawings are not comparable, and the bid comparison worksheet exists because that is the most common way homeowners lose money at this stage.

Go deeper

The evidence behind this section

ResearchOriginal analysis

StatisticsSourced figures

ToolsRun your numbers

Questions

Questions people ask

More in the full FAQ.

How much does an ADU cost?

It depends on type, size, site conditions and jurisdiction, and any single national figure will be wrong for your project by a wide margin. Our cost calculator builds a range from line items you supply for your own site. For sourced California evidence on what builders filed with cities, see our permit valuation research.

Why will you not just give me a price per square foot?

Because cost per square foot on an ADU is dominated by things that are not square footage — the foundation, the utility runs, the electrical service and the site access. A per-square-foot figure hides exactly the variables that decide your project.

What is the cheapest size to build?

In California, staying at or under 750 square feet of interior livable space exempts the unit from impact fees under Gov. Code § 66311.5. That is a genuine, sourceable saving. Whether the smaller unit suits your use is a separate question from whether it is cheaper.

Sources

  1. California Government Code §§ 66310–66342 (State ADU Law) Retrieved 2026-09-05 · Recodified from § 65852.2 by SB 477 (2024); fee section renumbered to § 66311.5 by SB 543 (2026).
  2. California ADU permit valuations (HyreADU analysis of jurisdiction permit records) Retrieved 2026-09-05 · A permit valuation is the applicant’s declared value of work, not a contracted bid.

Last reviewed 2026-09-16

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A straight answer: our ADU professional matching is still being built, so this is not a quote service, and we cannot tell you whether your lot qualifies. We can help you work out which questions to ask and what a thorough proposal should contain. Your planning department decides what is permitted at your address.

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