800 Sq Ft ADU: California’s By-Right Number, and What It Buys
Eight hundred square feet is the size California guarantees — and the last stop before development impact fees. Here is what that means for your layout and your budget.
An 800 sq ft ADU is the largest unit California guarantees a permit for on qualifying lots — cities must approve it ministerially, with 4-ft setbacks and at least 16 feet of height. It comfortably holds a two-bedroom, one-bath layout. At our published 2026 rates a custom detached build typically runs $215K–$305K. One budget note: ADUs under 750 sq ft are exempt from development impact fees; at 800 you cross that line and fees may apply, in proportion to the primary home’s.
The numbers at a glance
| The guarantee | Cities must approve up to 800 sq ft on qualifying lots |
| Typical layout | Two bedrooms, one bath |
| Custom detached (2026) | $215K–$305K at published rates |
| Impact fees | Exempt under 750 sq ft; proportional fees may apply at 750+ |
| Setbacks & height | 4-ft side and rear; at least 16 ft of height |
Browse by size: 400 sq ft · 500 sq ft · 800 sq ft · 1,200 sq ft — or see our floor plans.
What this size costs, by build type (2026)
| Build type | Typical 2026 range |
|---|---|
| Garage conversion (oversized/3-car garages, where the footprint exists) | $90K–$150K |
| Attached ADU (at published $260–$400/sq ft) | $210K–$320K |
| Custom detached ADU (new backyard build) | $215K–$305K |
Ranges at our published 2026 rates; final pricing depends on site conditions — panel, utilities, slope, access. Run your exact size in the cost calculator.
The guaranteed size
State law is explicit: a local agency must approve an ADU of up to 800 sq ft, at least 16 feet tall, with 4-ft side and rear setbacks, regardless of local floor-area ratios or lot-coverage rules that would otherwise block it. That makes 800 the by-right ceiling — the size you can plan around before any city-specific conversation. Larger units (up to the typical 1,200 sq ft) are entitled on most lots too, but 800 is where the state’s strongest guarantee sits.
What 800 square feet holds
This is genuine two-bedroom territory: two real bedrooms, one full bath (two if you spend the space), and an open kitchen-living area that doesn’t feel negotiated. Our Tustin (750 sq ft, 2 bed) sits just under this size, and our two-bedroom collection shows how the layouts trade space between bedrooms and living area. Two bedrooms widen who can rent the unit — families, roommate pairs, a parent plus a caregiver.
The 750 sq ft fee line
California exempts ADUs under 750 sq ft from development impact fees entirely; from 750 up, fees are allowed but must be proportional (by square foot) to what the primary house would pay. Whether the extra ~50–100 sq ft is worth the fee depends on your city’s fee schedule and what the space buys you — this is exactly the kind of trade we run in the first design conversation, with the calculator covering the construction side ($215K–$305K typical at 800 sq ft).
Frequently asked questions
Why is 800 sq ft called the "by-right" ADU size?
Can an 800 sq ft ADU have two bedrooms?
Does an 800 sq ft ADU pay impact fees?
How much does an 800 sq ft ADU cost to build?
Do I need to add parking for an 800 sq ft ADU?
Can I pair an 800 sq ft ADU with a JADU?
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