How much does an ADU cost in California?
An ADU in California costs $100 to $300 per square foot on HomeGuide's figures, with no state impact fee at 750 square feet or less.
The build price is national. What is specific to California is the fee rule, and it is written into the state code rather than set by each city: Government Code section 66311.5 says a local agency, special district or water corporation shall not impose any impact fee on an accessory dwelling unit with 750 square feet of interior livable space or less, or on a junior accessory dwelling unit of 500 square feet or less. Above 750 square feet, the section requires any impact fee to be charged proportionately against the square footage of the primary dwelling. The Department of Housing and Community Development states the same rule in its own housing-law fact sheets and publishes the ADU Handbook that local ordinances are measured against.
Sources: California Government Code section 66311.5, California Department of Housing and Community Development
This page is general information, not professional advice. Remodeling prices move with your local labor market, the condition of what is already there, the finishes you pick and what your building department requires, and the only number that counts is a written quote from a contractor who has walked the job.
Why 750 square feet is the number to design around
Because it is the only place in this whole subject where a single square foot changes the rules rather than the price. Under section 66311.5 an ADU at 750 square feet of interior livable space carries no impact fee at all; the same unit drawn at 800 square feet is inside the proportional-fee regime. The statute also says an ADU is not treated as a new residential use for calculating utility connection fees or capacity charges unless it was built alongside a new single-family home, and that a unit under 500 square feet of interior livable space is treated, for school-fee purposes, as construction that does not add 500 square feet of assessable space.
Interior livable space is the measurement the statute names. Ask your designer which number on the drawings that is before you assume the unit is under the threshold.
What the build itself costs at those sizes
| ADU size | Average cost |
|---|---|
| 400 sq ft | $40,000 – $120,000 |
| 500 sq ft | $50,000 – $150,000 |
| 600 sq ft | $60,000 – $180,000 |
| 750 sq ft | $75,000 – $225,000 |
| 1,000 sq ft | $100,000 – $300,000 |
| 1,200 sq ft | $120,000 – $360,000 |
Source: HomeGuide
HomeGuide's bands are national and it does not publish a California multiplier, so neither does this page. What it does publish is the split that decides the number: $80 to $160 per square foot for a prefab modular unit, $125 to $225 attached, $150 to $250 detached, and $50 to $200 for a garage conversion. On its table a 750-square-foot detached ADU reads $112,500 to $187,500 — the state's fee threshold sitting in the middle of a six-figure build either way.
Source: HomeGuide
The costs the fee rule does not touch
Section 66311.5 exempts impact fees, and it says in its own words that impact fee does not include a connection fee or capacity charge. Building permits are a separate line again — HomeGuide puts them at $500 to $2,000 nationally — and so is everything before the permit: house plans at $1,200 to $5,000, a land survey at $400 to $1,800, excavation at $1,500 to $5,000, grading at $1,000 to $5,000 and a foundation at $2,400 to $10,500. A fee exemption is worth real money; it is not the whole soft-cost line.
Sources: California Government Code section 66311.5, HomeGuide
To price a California ADU size against the published build tables: the home addition cost calculator.
California ADU rules are state law but the crews are local. These are the general contractors AI assistants actually name when homeowners ask. Nobody can pay to be on these lists.
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Questions people ask next
- Are ADUs exempt from impact fees in California?
- California Government Code section 66311.5 bars a local agency, special district or water corporation from imposing any impact fee on an ADU of 750 square feet or less of interior livable space, or a junior ADU of 500 square feet or less. Above 750 square feet, fees must be charged proportionately against the primary dwelling's square footage.
- How much does a 750 sq ft ADU cost in California?
- HomeGuide's national band puts a 750-square-foot ADU at $75,000 to $225,000, or $112,500 to $187,500 at its detached rate of $150 to $250 per square foot. It publishes no California multiplier, so no page should apply one.
- Does an ADU count as a new residential use for water and sewer fees?
- Not under section 66311.5, which says an ADU or junior ADU shall not be considered a new residential use for calculating connection fees or capacity charges for utilities including water and sewer, unless the unit was constructed with a new single-family dwelling.
- Where do I check my city's ADU rules?
- The California Department of Housing and Community Development publishes the ADU Handbook that state ADU law is read through, and runs the review process local ADU ordinances are submitted to. Your own city's planning department applies it to your lot.
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