1200 square foot ADU: San Diego's largest ADU, plans and cost

Is a 1200 square foot ADU the largest in San Diego?

Yes, for new construction. San Diego Municipal Code section 141.0302 allows an ADU from 150 to 1,200 sq ft. An ADU converted from space that already exists, such as part of the house or a garage, has no maximum size. The County of San Diego uses the same 1,200 sq ft cap for detached ADUs in unincorporated areas. Other cities in the county set their own limits, so check yours.

State law sets a floor, not a ceiling. A city can't cap ADUs below 850 sq ft (1,000 for more than one bedroom), and it must approve an 800 sq ft detached ADU with 4-foot setbacks. San Diego goes well past both. Since 2026 the state measures ADU size as interior livable space, while the city's code and bulletin speak of gross floor area, so confirm how your designer is counting before you draw right up to 1,200. For the rest of the rules, see California ADU laws.

1200 sq ft ADU plans

The County of San Diego publishes three free plans at exactly 1,200 sq ft. They're the clearest picture of what fits at this size:

The “best for” column is our read, not the County's. The plans are about 85% complete; a designer still adds the site plan, stormwater documents, energy calculations and truss design. The City of San Diego accepts the County's plans for a 30-day review.

3-bedroom options

If three bedrooms is the goal, you have more than one free starting point. Besides the County's Plan A, Escondido's Plan 4 can be built as three bedrooms with two baths in just 1,000 sq ft, and Encinitas's permit-ready plans run up to three bedrooms. Our ADU floor plans guide compares every size, and pre-approved ADU plans in San Diego covers how each program works.

A 1,200 sq ft single-story unit takes a lot of yard. San Diego's code allows up to two stories in single-family zones, which halves the footprint. See our two-story ADU guide.

1200 sq ft ADU cost

Snap ADU's published San Diego prices (March 2026) put a detached 1,200 sq ft ADU at about $450,000, or about $375 per square foot. That's the lowest per-foot price on its list, because design, permits and utility work cost about the same at any size. The base includes design, permit management, about $35,000 of level-lot site work and standard finishes; it excludes fees, demolition, retaining walls and a separate meter.

Per-foot figures are our division of Snap's prices. Lars Remodel's October 2026 range for a detached ADU in San Diego, $250,000 to $450,000 or more, puts 1,200 sq ft at or above the top end. Your price depends on the lot, utility runs and finishes. Try our ADU cost calculator and read what an ADU costs in San Diego for site costs that change the total.

Fees at 1,200 sq ft

In Snap ADU's February 2026 table of City of San Diego fees, a 1,199 sq ft ADU comes to about $31,111, up from $26,430 at 999 sq ft. Those are a builder's estimates, not the city's fee schedule.

• Impact fees. State law bars impact fees only for ADUs of 750 sq ft or less. Above that, they must be proportional to the main house. The City of San Diego goes further and exempts the first two ADUs on a property from development impact fees at any size.
• From 2027. SB 1117 limits impact fees on an ADU over 750 sq ft to the area above 750, so a 1,200 sq ft unit is charged on 450 sq ft. That matters most outside the City of San Diego. See California ADU laws for 2027.
• School fees. San Diego Unified charges $5.38 per sq ft, about $6,456 on 1,200 sq ft.

Floor area above 800 sq ft

In the City of San Diego, this rule can trim a 1,200 sq ft plan. The city's ADU bulletin says an ADU's floor area counts toward the lot's floor area ratio (FAR), the cap on total building area for the lot. The exception is one ADU of 800 sq ft or less, which can be built even if it puts the lot over its FAR limit.

That means an ADU of up to 800 sq ft is safe whatever the house already uses, but a 1,200 sq ft ADU has to fit within the floor area your lot has left. On a small lot with a large house, that room may not exist. State law protects only the 800 sq ft unit from lot coverage, FAR and open space limits.

One big ADU or two smaller ones

The same 1,200 sq ft could be one three-bedroom unit or two smaller homes. Today, state law guarantees one new detached ADU on a single-family lot, plus a conversion ADU and a junior ADU. AB 956 raises that to two detached ADUs starting January 1, 2027. Our guide to how many ADUs you can build covers every combination, including San Diego's bonus program.

Prices from Snap ADU's price list and fee table; a builder pricing two units as one project may quote less than the sum. School fees under 500 sq ft are waived under state law. Some rules of thumb:

• Build one 1,200 sq ft unit if it's for a family, a parent with a caregiver, or you plan to move into it yourself and rent the house.
• Build two smaller units if rental income is the goal and the lot can take both, since two rents can beat one. Run the numbers in our ADU rental income guide.
• Mind the third ADU. San Diego's development impact fee exemption covers the first two ADUs on a property, so a third would pay.

Whichever you choose, the builder matters as much as the plan. Compare verified ADU builders in San Diego, see the best ADU builders in San Diego, check a contractor's license and read how to hire an ADU contractor.

Common questions