ADU floor plans and sizes: what fits in 400 to 1,200 square feet

ADU floor plans by size

The best guide to what fits in a given size is a plan a building department has already checked. The sizes below come from the free plan sets published by the County of San Diego and the City of Escondido. Your designer can lay out the same space differently, but these give you an honest baseline.

Sources: County of San Diego and City of Escondido plan listings. Since January 1, 2026, ADU size limits in state law are measured as interior livable space, so check how a plan's square footage is counted before you compare it to a limit.

Studio ADU and the 400 sq ft ADU

A studio puts living, sleeping and kitchen in one room with a separate bathroom. Escondido's smallest plan, at 484 sq ft, is drawn to work either as a studio or as a one-bedroom. The City of San Diego allows an ADU as small as 150 sq ft, so a studio well under 400 is legal there, though it's tight for a long-term tenant. If you're carving space out of your existing house rather than building new, a junior ADU is capped at 500 sq ft and can share the main home's bathroom.

1 bedroom ADU floor plans: the 500 sq ft and 600 square foot ADU

Somewhere around 500 to 600 sq ft, a bedroom gets its own door. The County's Plan F packs a living room, kitchen, bedroom, bathroom and a laundry room into 600 sq ft. A 500 sq ft ADU has a particular draw: units under 500 sq ft owe no school fees. San Diego Unified charges $5.38 per sq ft, or about $2,700 on a 500 sq ft unit.

2 bedroom ADU floor plans and the 800 sq ft ADU

Two bedrooms can fit in less space than most people expect. Escondido's Plan 2 does it in 644 sq ft with one bathroom. The County chose to use its 800 sq ft plan, Plan E, for a single larger bedroom instead. Around 800 sq ft is worth a close look because state law guarantees you can build a detached ADU of at least that size with 4-foot side and rear setbacks, even where local rules would otherwise squeeze it.

2 bedroom, 2 bath ADUs around 1,000 sq ft

In the free public plan sets, a second full bathroom first shows up at 851 sq ft. Escondido's Plan 3 fits two bedrooms and two baths in 851 sq ft, and its Plan 4 can be built as two or three bedrooms with two baths in 1,000 sq ft. The County's 1,000 sq ft Plan D takes a different approach: one bedroom with one and a half baths.

The 1,200 square foot ADU

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. The County publishes three free plans at exactly that size: three bedrooms with one bath, two bedrooms with one bath, and two bedrooms with two baths. Conversions of existing space in the City of San Diego have no maximum size.

Size lines that change your costs

Square footage matters most where it crosses a legal line. These are the ones to plan around:

Sources: state statutes, HCD and local codes. In the City of San Diego, the first two ADUs on a lot are exempt from development impact fees regardless of size, so the 750 line matters less there than in other cities. School fees and plan check fees still rise with size. For the full set of rules, read our guide to California ADU laws.

Free ADU plans you can download

Several San Diego County governments publish ADU house plans you can download and use. They're a good way to see real floor plans at each size, even if you end up designing your own.

• County of San Diego. Plans A through F above, free, in PDF and CAD formats. They're about 85% complete; you or your designer add the site plan, stormwater documents, energy calculations and truss design. Download the County plans.
• City of Escondido. Four free plans from 484 to 1,000 sq ft. They're for new, single-story, detached ADUs only. You can change select features like windows and doors, but not the size, shape or layout. See Escondido's PAADU plans.
• City of Encinitas. Permit-ready plans for detached ADUs, from a studio to three bedrooms. See Encinitas's PRADU program.
• City of Chula Vista. Standard ADU plans drawn by private designers and approved by the city. To use one, you contract with the designer who owns it.

The City of San Diego accepts the County, Chula Vista and Encinitas plans for a 30-day review, as long as the plan meets the city's code. Our guide to pre-approved ADU plans in San Diego covers what each program includes and what you still have to add.

ADU layouts that work

Small floor plans succeed or fail on a few decisions. These are practical rules of thumb designers use, not code requirements.

• Stack the plumbing. Putting the kitchen and bathroom back to back, or the bathroom next to the laundry, keeps drain and water lines short. That saves money and frees up wall space elsewhere.
• Give the bedroom its own wall from the living room. In a one-bedroom, a closet or bathroom between the bedroom and living space blocks noise better than a single wall.
• Plan laundry from the start. The County's 600 sq ft plan includes a laundry room, a sign that even small units can fit one. A stacked washer and dryer in a closet works where space is tighter.
• Use height when you can't use width. State law lets a detached ADU be at least 16 feet tall, and taller near transit. A vaulted ceiling in the main room makes a small plan feel bigger without adding square footage or fees.
• Count storage early. Small units often lose closets to make rooms bigger. Built-ins, a pantry wall and space under a window seat do more than an extra foot of floor.
• Think about the yard door. Where the entry faces affects privacy for you and your tenant. A separate path from the street is worth sketching before the floor plan is final.

Choosing an ADU size

Bigger costs more, but not proportionally. Snap ADU's published San Diego prices (March 2026) show the jump from 500 to 1,200 sq ft adds 140% more space for about 50% more money, because fixed costs like design, permits and site work don't scale with size.

Sources: Snap ADU's published prices and fee table, not official city figures. The biggest fee jump comes between 499 and 749 sq ft. For a full breakdown, see what an ADU costs in San Diego or try our ADU cost calculator.

A few ways to decide:

• For rental income, compare what one- and two-bedroom units rent for near you against the added build cost. Our guide to ADU rental income in San Diego walks through the math.
• For a parent or one adult, a 500 to 600 sq ft one-bedroom keeps fees low and is easier to keep up.
• For a family, look at plans from 1,000 to 1,200 sq ft with two baths.
• If your lot is tight, start from the 800 sq ft guarantee and 4-foot setbacks, or consider an attached unit. See attached vs. detached ADUs.

Whatever size you pick, the builder matters as much as the floor plan. Compare verified ADU builders in San Diego, check a contractor's license and read how to hire an ADU contractor before you sign.

Common questions