Check the original sources and current program guidance before making a property-specific decision.
How much does an ADU cost per square foot in California?
There is no single statewide rate. As a local benchmark, SnapADU’s March 2026 San Diego examples for 500–1,200 sq ft detached homes work out to $300–$440 per sq ft for construction and $375–$600 per sq ft for its stated all-in scope. These are one builder’s examples, not California averages or upper price limits.
Source checked September 26, 2026. The builder describes its all-in figures as including design, permits, sitework and utilities. Confirm property-specific exclusions and contingency with the builder; the figures are not completed-project invoices or quotes for your lot.
ADU.app divides the published totals by floor area and rounds to whole dollars. This makes the 750 sq ft row $367 and $467 per sq ft; the source presents those rates more approximately.
| Floor area / baths | Construction | Construction / sq ft | Builder all-in | All-in / sq ft |
|---|---|---|---|---|
| 500 sq ft / 1 bath | $220,000 | $440 | $300,000 | $600 |
| 750 sq ft / 1 bath | $275,000 | $367 | $350,000 | $467 |
| 1,000 sq ft / 2 bath | $335,000 | $335 | $425,000 | $425 |
| 1,200 sq ft / 2 bath | $360,000 | $300 | $450,000 | $375 |
What do 500, 800 and 1,200 sq ft ADUs cost in the calculator?
The examples below use the current ADU.app calculator with identical assumptions. They are reproducible planning scenarios, separate from the builder prices above.
Model v2.1 assumptions: detached ADU, California — other preset, one bathroom, standard finishes, one story, typical sitework, moderate utility upgrades, no extras, no quote overrides and a 12% contingency. The geographic preset is an editorial assumption, not measured local pricing.
The planning total includes construction, design, permit, sitework, utility and contingency allowances. Land and financing costs are excluded. Other property-specific items may require an extra allowance. The low–high range is a sensitivity band, not a statistical confidence interval or guaranteed price.
Calculation: planning total ÷ floor area = effective project cost per square foot. For your own comparison, divide the construction line alone by floor area to obtain a construction-only rate. This article now uses model v2.1; older v1.1 examples remain labeled in other articles and in the methodology archive.
| Size | Planning total | Planning range | Project cost / sq ft |
|---|---|---|---|
| 500 sq ft | $257,085 | $237,000–$288,000 | $514 |
| 800 sq ft | $347,547 | $320,000–$389,000 | $434 |
| 1,200 sq ft | $464,400 | $428,000–$520,000 | $387 |
Why does a small ADU cost more per square foot?
A small ADU still needs expensive fixed components, so those costs are divided by fewer square feet.
A 500-square-foot unit still needs a kitchen, bathroom, water heater, electrical distribution, heating and cooling, professional plans, permits, and utility connections. Increasing floor area may add framing and finishes without duplicating every fixed system.
That does not mean a larger ADU is cheaper overall. It means the effective all-in rate can decline as floor area increases, while the total project cost rises.
Which costs should be inside the calculation?
Include design and engineering, permits, site and utility work, complete construction, owner items, and a visible contingency before calculating an all-in rate.
Ask every bidder to price from the same drawings, specification, allowance schedule, and written inclusions. Separate unknowns instead of hiding them in a single rate.
- Define whether the rate is building-only or complete-project.
- List design, permit, utility, delivery, tax, and landscaping exclusions.
- Use verified site information and realistic allowances.
- Carry contingency separately from finish upgrades.
- Compare at least two proposals using the same scope.
How do you turn a rate into a property-specific budget?
Select your location and building approach, then replace the calculator’s six cost categories with written local quotes or confirmed fees where available.
Start with construction, design and engineering, permits, sitework, utilities and extras. A quote override replaces its entire category; check the scope before entering it so you do not count the same work twice. Keep a separate contingency for unresolved risks.
For rules and permit research, start with California HCD’s ADU Handbook and the city or county responsible for your parcel. HCD is a source for the state framework, not an endorsement of these prices. Ask the building department and utility providers for current property-specific fees and connection requirements.
Compare two or three scenarios and save the assumptions with your budget. A chosen city, unit size or construction type in the calculator does not establish that the proposal is permitted on your property.
COMMON QUESTIONS
Frequently asked questions
Direct answers to related questions, with important limits and verification steps kept visible.
Are these California ADU prices contractor quotes?+
No. The first table is one San Diego builder’s published pricing examples; the second uses ADU.app’s editorial calculator assumptions. Neither is a quote for your property or a statewide market average.
Should permits be included in cost per square foot?+
They should be included if the number is described as an all-in project rate. Always state the scope so two figures can be compared fairly.
Does a prefab ADU have the same rate?+
Not necessarily. The factory package, site work, delivery, foundation, installation, utilities, permitting, and owner costs must all be combined before comparison.
OFFICIAL AND PRIMARY SOURCES
Sources and methodology
Rules and program guidance can change. Review the latest official material and confirm property-specific requirements with the responsible agency and qualified professionals.
This article provides general educational information and does not replace property-specific legal, financial, tax, design, engineering, appraisal, lending, or construction advice. ADU.app’s editorial team writes the article; the research-review role checks citations and calculation consistency. Last updated September 26, 2026. Confirm current requirements with official sources.