3 bedroom ADU: floor plans, size limits and costs

Does a 3 bedroom ADU fit under 1,200 sq ft?

It does, and the County of San Diego proves it with a free plan. The County's Plan A is a 1,200 sq ft three-bedroom with one bath. That size isn't an accident: 1,200 sq ft is the largest new ADU the City of San Diego allows, and the County uses the same cap for detached ADUs in unincorporated areas.

Builders go smaller. Snap ADU's 798 sq ft L-shaped plan fits three bedrooms, one central bath, an open great room and a separate laundry room. Two bedrooms sit together at one end and the third forms the short leg of the L. At that size each bedroom is close to the code minimum, so it works best for children or a home office. For a quick comparison of studios through three bedrooms, see our ADU floor plans overview.

Since January 1, 2026, state ADU size limits are measured as interior livable space, so ask your city how it counts a plan's square footage before you push a three-bedroom right up to the cap.

3 bedroom ADU floor plans

Public agencies publish only a few three-bedroom ADU plans. Builders offer many more: Snap ADU's three-bedroom catalog lists 22, from 798 to 1,500 sq ft (the largest include a garage).

Sources: Snap ADU plan pages, Escondido's PAADU plans, La Mesa's pre-approved plans and the County plan list. Encinitas's permit-ready plans also run up to three bedrooms.

1,200 sq ft ADU plans with 3 bedrooms

At the cap, the extra space buys a second bath. Snap ADU's 1,199 sq ft plan has two bedrooms with their own bathrooms, a third near the entry, an open kitchen and living area, in-unit laundry and a covered porch. The County's Plan A spends the same 1,200 sq ft on three bedrooms with a single bath, which costs less to plumb but is harder on a family of five in the morning.

Around 1,000 sq ft

Snap ADU's 999 sq ft L-shaped plan has a primary suite with a large closet, two bedrooms sharing a hall bath and a kitchen with room for an 8-foot island. It's listed as pre-approved in Chula Vista, Poway, San Diego and San Marcos. Staying at 999 sq ft also keeps a three-bedroom legal in cities that cap ADUs at 1,000.

How big a city lets you build

The size cap, not the floor plan, is usually what decides whether a three-bedroom works. Under Government Code section 66321, a city's maximum ADU size can't be set below 1,000 sq ft for a unit with more than one bedroom. Most San Diego County cities allow 1,200, but not all.

Sources: San Diego's municipal code, County of San Diego, Vista's ADU page and Ordinance 2024-02, Poway's ADU information sheet, Coronado's code and Solana Beach's code. Other cities set their own numbers; find yours on our San Diego ADU builders and city rules pages, and read California ADU laws for the state baseline.

What makes a room a bedroom: size and egress rules

Three bedrooms means three sets of code requirements. These come from Chapter 3 of the 2025 California Residential Code, in effect since January 1, 2026:

• The escape window drives the layout. Every bedroom needs an exterior wall big enough for a code window. In a long, narrow plan, an interior bedroom with no outside wall won't pass.
• Windows near a property line are limited. Fire rules restrict openings in walls close to a lot line, so check where each escape window can go if the ADU sits near the fence. Our ADU building code guide covers fire separation.
• Conversions are harder. A garage or basement often lacks windows that meet the size and sill limits, so a three-bedroom conversion may need new openings and headers.
• A loft isn't automatically a bedroom. Lofts have their own code section and limits. See our ADU with loft guide before counting one as a room.

3 bedroom ADU cost

Builder plan prices cover the building and its finishes only. Plans, permits, fees and site work come on top.

Sources: Snap ADU plan pages. The 1,199 sq ft plan's page says the all-in cost on an ideal lot is in the “low $400Ks.” Snap ADU's March 2026 San Diego estimates put a detached 1,000 sq ft ADU at about $425,000 and 1,200 sq ft at about $450,000 all-in. Lars Remodel's October 2026 range for detached ADUs is $250,000 to $450,000 or more.

Sources: Snap ADU's fee table (a builder's estimate, not official city figures) and San Diego Unified. Every three-bedroom ADU is over 750 sq ft, so outside the City of San Diego it may owe impact fees proportional to the main house. Inside the city, the first two ADUs are exempt from city impact fees. Estimate your own project with the ADU cost calculator, and see what an ADU costs in San Diego.

What a 3 bedroom ADU rents for

The 2026 fair market rent for a three-bedroom in San Diego County is $3,998 a month. Snap ADU estimated in October 2025 that a new 1,000 sq ft three-bedroom ADU rents for about $4,500; that's a builder's figure, not a survey. More in our ADU rental income guide.

A 3 bedroom ADU for a family

A three-bedroom ADU is usually a family home, either for a grown child's household, for parents moving in with a caregiver, or as a rental for a family. A few things matter more for a family than for a single renter:

• Two baths. One bath for three bedrooms is legal (County Plan A does it), but two is what most families expect.
• Laundry and storage. Both public and builder plans at this size include in-unit laundry; insist on it.
• A bedroom on the ground floor if a grandparent may live there. Snap ADU's catalog includes two-story 1,155 sq ft versions with all three bedrooms upstairs or with one on the ground floor.
• No owner-occupancy rule. State law doesn't let a city require you to live on the property to have an ADU, so you can rent the house or the ADU. In the City of San Diego, ADU rentals must be for at least 31 days.

The free County Plan A is about 85% complete, and the City of San Diego accepts County plans for a 30-day review. See pre-approved ADU plans in San Diego. A three-bedroom is the biggest ADU you can build, so the builder matters: 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