800 sq ft ADU: the size California guarantees, with costs

Why 800 sq ft is the size California guarantees

On a single-family lot, state law requires every city to approve one new detached ADU of up to 800 sq ft with 4-foot side and rear setbacks, along with a conversion ADU and a junior ADU. The companion rule in Government Code § 66321 stops a city from using lot coverage, floor area ratio, open space or front setback rules to block an ADU of at least 800 sq ft with those setbacks. Other local standards, like height, still apply.

Two more rules make 800 sq ft safe everywhere:

• Size caps can't go lower. Under the same section, a city's maximum ADU size can't be set below 850 sq ft, or 1,000 sq ft for more than one bedroom. So no city's size limit can stop an 800 sq ft unit. The City of San Diego's limit is 1,200 sq ft for a new ADU.
• Front setbacks count too. HCD told San Mateo in 2024 that an 800 sq ft ADU with 4-foot side and rear setbacks can't be denied just because it's in the main house's front setback.

How big is an 800 sq ft ADU?

Eight hundred square feet is a 20-by-40-foot rectangle, or a square about 28 feet on each side. This Old House puts a two-car garage at about 400 to 440 sq ft, so an 800 sq ft ADU is about two two-car garages side by side.

That's a full apartment. RentCafe's apartment-size analysis puts the average U.S. one-bedroom at 735 sq ft and the average two-bedroom at 1,095. An 800 sq ft ADU is a little bigger than a typical one-bedroom and a compact two-bedroom.

Since January 1, 2026, the 800 is interior livable space, so the building's outside footprint is a bit larger.

800 sq ft ADU floor plans

At 800 sq ft you can choose a generous one-bedroom or a two-bedroom. Free public plans show both:

Sources: the County of San Diego's free plans and Escondido's pre-approved plans. If two bedrooms fit in 644 sq ft, 800 leaves room for a bigger living area or laundry room. A second full bathroom first shows up in these public sets at 851 sq ft. Builders do squeeze two baths into 800: Snap ADU's price list includes an 800 sq ft two-bed, two-bath unit as part of a two-ADU package.

2 bedroom ADU floor plans at 800 sq ft

A common 800 sq ft two-bedroom arrangement:

• Center: an open kitchen, dining and living room across the middle of the plan.
• Each end: one bedroom, so neither shares a wall with the other. This split layout suits roommates or a parent and adult child.
• Bath and laundry: off a short hall next to one bedroom, with the kitchen backing onto the same plumbing wall.

The County's Plan E shows the other choice: one larger bedroom, which suits a single owner downsizing or a parent who wants space. Compare every size in ADU floor plans, or step up to a 600 square foot ADU if a one-bedroom is enough.

Just over the 750 sq ft fee line

State law bars impact fees on ADUs of 750 sq ft or less. Above 750, a city or special district may charge them, but only in proportion to the square footage of the main house. At 800 sq ft, you're 50 feet past that line.

Sources: SDMC § 142.0640, the Governor's signing release for SB 1117, the state ADU handbook on school fees, San Diego Unified's rate and Snap ADU's fee table, a builder's estimate rather than an official city schedule.

So does 800 cost more in fees than 750? In the City of San Diego, barely. The impact-fee exemption covers your first two ADUs, and in Snap ADU's table the only line that grows between 749 and 999 sq ft is the school fee. At San Diego Unified's rate, 50 extra feet add about $270. Outside the city, in a jurisdiction that does charge ADU impact fees, ask for the number before you settle on 800 versus 750.

800 sq ft ADU cost

Snap ADU's published San Diego prices (March 2026) don't list a stand-alone 800 sq ft unit, but they bracket it:

Those all-in figures cover design, permits, base sitework and utilities, and leave out city fees, upgrades and site extras. Snap ADU's sitework guide prices the common extras: a separate SDG&E meter at $10,500 to $15,000, a 2- to 4-foot slope at $10,000 to $20,000, and retaining walls at $20,000 to $30,000 or more. A second bathroom moves you toward the top of the range.

See what an ADU costs in San Diego for every size, and test your lot in the ADU cost calculator.

Fitting 800 sq ft on your lot

The guarantee gets you 4-foot side and rear setbacks, but a few other rules shape where 800 sq ft actually goes:

• Height. A detached ADU can be at least 16 feet tall under state law, and taller near major transit.
• San Diego's low-profile bonus. In the City of San Diego, an ADU 16 feet tall or less needs no interior side or rear setback, except 4 feet in high and very high fire hazard zones. The city also allows two-story ADUs in single-family zones, which lets 800 sq ft sit on a 400 sq ft footprint. See the two-story ADU guide.
• Fire walls. Under the California Residential Code, walls less than 3 feet from the property line can't have windows or doors, and walls within 5 feet generally need a fire rating.

If a detached unit won't fit, attaching the ADU to the house is another route; see attached vs. detached ADUs for the trade-offs.

Renting an 800 sq ft ADU

The second bedroom is where 800 sq ft pays off. HUD's 2026 fair market rents for San Diego County are $3,001 a month for two bedrooms, against $2,459 for one, a $542 gap. Fair market rents describe modest units with utilities included, so treat them as a benchmark.

Our guide to ADU rental income in San Diego runs the payback. If a smaller unit fits your plans better, see the small ADU guide. When you're ready for bids, compare the best ADU builders in San Diego and check a contractor's license before you sign.

Common questions