ADU Cost Calculator for California
Build cost · Rental income · Payback
For a 600 sq ft detached ADU, this planning model estimates $231,000 to $289,000 in build cost, including an allowance for design and permit soft costs. Using county HUD rent assumptions, modeled simple payback spans about 10 to 13 years across the three counties. These are estimates, not local contractor quotes or guaranteed returns.
Use the calculator free, without an account. Change the county, size, finish and rent assumption to compare scenarios. Los Angeles, San Diego and Santa Clara have separate rent anchors; construction uses the same national cost base. Financing and unusual site work are excluded.
Estimate your ADU
- Build cost
- $231K–$289K
- Rent per month
- $2,328
- Simple payback
- 13.3 years
A 600 sq ft ADU in Los Angeles County nets about $18,857 a year after operating costs. These are countywide estimates, not a quote for your lot.
Itemized budget breakdown
Construction and soft costs itemize the model without adding fees twice. Site work and financing are unpriced until you have site-specific quotes.
- Construction (hard costs)
600 sq ft · Cedar · openings ($9K)
$213,000Model allowance
- Design & permits (soft costs)
18% allowance for architectural plans & municipal fees
$38,34018% allowance
- Site work
Grading, utility trenching, tie-ins, foundation prep
UnpricedSite-specific
- Financing & loan costs
Interest during build, origination, closing
UnpricedLoan-dependent
- Itemized total midpoint$251,340
Free address check
Countywide tools can't check lot boundaries or setbacks. Enter your California address to see property lines, base-zone setbacks, and a placed ADU in 3D.
Compare two sizes before you commit
Start with 600 sq ft and Conservative (HUD rent), then try 800 sq ft in the same county. Both use a one-bedroom rent anchor, so a larger footprint can raise cost without raising the modeled rent. Next, change the rent assumption to see how much your payback depends on it. Check current comparable rentals before relying on the higher scenarios.
Simple payback divides the modeled construction midpoint by annual rent after operating expenses. It is not a mortgage cash-flow forecast: loan payments, taxes on income and future rent changes need a separate review.
Modeled build cost by unit size
Build cost by size, cedar finish and shed roof. Build cost uses the same national cost base in every county, so one table covers all three. For permit-fee context, scope exclusions, and bid guidance, read the California ADU cost guide.
| Unit size | Estimated build cost | Per sq ft |
|---|---|---|
| 400 sq ft | $157K–$197K | $393–$493 |
| 600 sq ft | $231K–$289K | $385–$482 |
| 800 sq ft | $305K–$381K | $381–$476 |
| 1,200 sq ft | $453K–$566K | $378–$472 |
ADU rental income by county
Monthly rent at the county's HUD Fair Market Rent: studio rent for 400 sq ft, one-bedroom for 600 and 800 sq ft, two-bedroom for 1,200 sq ft. Market rents in desirable neighborhoods often run higher.
| County | 400 sq ft | 600 sq ft | 800 sq ft | 1,200 sq ft |
|---|---|---|---|---|
| Los Angeles County | $1,979 | $2,328 | $2,328 | $2,957 |
| San Diego County | $2,090 | $2,459 | $2,459 | $3,123 |
| Santa Clara County | $2,535 | $2,982 | $2,982 | $3,787 |
ADU payback and ROI
Simple payback is the midpoint build cost divided by a year of rent after operating costs. It ignores financing, appreciation, and tax effects, so treat it as a comparison between options, not a forecast.
| County | 400 sq ft | 600 sq ft | 800 sq ft | 1,200 sq ft |
|---|---|---|---|---|
| Los Angeles County | 10.7 yrs | 13.3 yrs | 17.6 yrs | 20.5 yrs |
| San Diego County | 10.1 yrs | 12.6 yrs | 16.6 yrs | 19.5 yrs |
| Santa Clara County | 8.3 yrs | 10.4 yrs | 13.7 yrs | 16.0 yrs |
Smaller units pay back faster because rent falls less than cost as size shrinks.
These are the rules. Here is what they mean for your lot.
Enter your California address to see rent, build-cost, and payback estimates for your specific parcel. Free to explore — no account needed for your first address.
What the estimate includes
- Hard construction cost of $315–$355 per sq ft depending on the exterior finish, plus a roof-style premium and $9,000 for windows and doors.
- A 18% allowance on top for soft costs such as design, engineering, and permits. The range runs from 8% below to 15% above that midpoint.
- Rent from the county's HUD Fair Market Rent, with 32.5% of it set aside for vacancy, management, maintenance, insurance, utilities, and property tax.
- Not included: land, unusual site work such as long utility runs or retaining walls, financing costs, and local fees above the state limits.
HUD FMR · Los Angeles County, CA · HUD FY2026 FMR (effective 2026-05-21; seed extract) · Medium confidence. The same HUD vintage backs every county row.
State rules that change your budget
California caps what a city can charge and require. ADUs of 750 sq ft or less of interior livable space are exempt from local development impact fees (school fees and utility connections remain separate), a city can require at most one parking space, and converting a garage or other existing space needs no new setbacks. The California ADU rules guide has the full state floor and city-by-city rules for San Diego County, including the City of San Diego.
These are the rules. Here is what they mean for your lot.
Enter your California address to see rent, build-cost, and payback estimates for your specific parcel. Free to explore — no account needed for your first address.
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