400 Sq Ft ADU: What Fits, What It Costs, and When Small Wins
The honest guide to a 400-square-foot ADU in Southern California — layouts that work, the 2026 build math, and why this size clears lots that bigger plans can’t.
A 400 sq ft ADU is a studio or compact one-bedroom — a real kitchen, a full bathroom, and one main living space. California guarantees a permit for an ADU of up to 800 sq ft on qualifying lots, so a 400 sq ft unit approves easily and fits where larger plans fail setbacks. Built as a custom detached unit it typically runs $200K–$230K at our published 2026 rates; a 2-car garage conversion of about the same size runs $90K–$150K.
The numbers at a glance
| Typical layout | Studio or compact one-bedroom |
| Custom detached (2026) | $200K–$230K at published rates |
| Garage conversion | $90K–$150K (a 2-car garage is ~400 sq ft) |
| Approval | Ministerial — 60-day clock, no hearing |
| Impact fees | None — ADUs under 750 sq ft are exempt |
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 (a 2-car garage is ~400 sq ft) | $90K–$150K |
| Junior ADU (within your existing house) | $60K–$130K |
| Custom detached ADU (new backyard build) | $200K–$230K |
Ranges at our published 2026 rates; final pricing depends on site conditions — panel, utilities, slope, access. Run your exact size in the cost calculator.
What actually fits in 400 square feet
Think of a generous hotel suite with a real kitchen: one main room that takes a bed or a sofa-bed arrangement, a kitchen wall or galley with full-size appliances, a full bathroom, and a closet. Done as a compact one-bedroom, the bedroom is small but private; done as a studio, the living area breathes. What does not fit: a second bedroom, a bathtub-plus-shower bathroom, or interior corridors — at this size, every hallway is a room you gave up.
Why small approves fast
State law requires cities to approve an ADU of up to 800 sq ft (up to 16 feet tall, with 4-ft side and rear setbacks) on qualifying lots — a 400 sq ft plan sits far inside every one of those limits. It is also the least site-hungry footprint we draw: on narrow side yards and small postwar lots, 400 square feet is often the difference between an ADU and no ADU. Our smallest standard plan, The Laguna (450 sq ft), lives in exactly this territory.
The cost math — small is not proportionally cheap
The kitchen and bathroom are the expensive rooms in any ADU, and a 400 sq ft unit has both. That is why the custom-detached estimate is $200K–$230K rather than half the price of an 800 sq ft build: the fixed costs — foundation, utility connections, kitchen, bath — do not shrink with the floor plan. The budget alternative at this size is a 2-car garage conversion (a 2-car garage is roughly 400 sq ft), typically $90K–$150K because the shell, slab and utilities already exist. Run your own numbers in the cost calculator.
Frequently asked questions
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