ADU floor plans · 400 sq ft
A 400 sq ft ADU is the smallest detached unit most California jurisdictions allow without a public hearing. At this footprint you can build a clean open-plan studio or a tight 1-bedroom — but every square foot has to work twice. These layouts show what's actually buildable and what compromises each layout asks of the occupant.
What fits in 400 sq ft: A small but real kitchen (apartment-size fridge, 2-burner cooktop, 5 ft of counter), a full bathroom with shower or compact tub, a sleeping area for one or two people, and a living/dining area that doubles as workspace. What doesn't fit: dedicated office, washer/dryer (combo units yes, separate no), guest space, large dining table.
The layouts below show the four most common ways to slice this footprint. None is universally “best” — the right one depends on whether you need a real bedroom door, how much storage you can build vertically, and whether you cook seriously or mainly heat prepared food.
Studio · 1 bath · Est. $120K – $150K
Best for: Single occupant or couple who prioritize cooking and entertaining. No bedroom door means lower privacy but maximum flexibility.
Studio + alcove · 1 bath · Est. $130K – $160K
Best for: Same square footage but partitions off the sleeping area for visual privacy. Sacrifices ~10% of living space.
1 BR · 1 bath · Est. $140K – $170K
Best for: Long, narrow lots. A true 1BR with a door — kitchen and living share the front of the unit, bedroom + bath at the back.
1 BR · 1 bath · Est. $145K – $180K
Best for: Squarer lots. Bedroom is perpendicular to living/kitchen, giving a better sense of separation than the linear plan.
Is a 400 sq ft ADU big enough to live in?
Yes — building codes treat 400 sq ft as a fully habitable dwelling unit if it has a kitchen, bath, and sleeping area. It works best for a single occupant or couple. Families don't typically fit comfortably below 600 sq ft.
How much does a 400 sq ft ADU cost to build?
Typically $120,000–$180,000 all-in including design, permits, and construction. Cost per square foot runs higher than larger ADUs (often $300–$450/sq ft) because fixed costs — permits, utility hookups, foundation — don't scale down with footprint.
Can I rent out a 400 sq ft ADU?
In California yes, in most jurisdictions. Studios and 1BRs at this size rent at the lower end of the local market — typically $1,200–$2,400/month depending on metro. Owner-occupancy requirements vary by city and were largely eliminated by state law in 2020, though some jurisdictions still apply them.
Will a 400 sq ft ADU need a separate permit?
Yes — every detached ADU requires a building permit regardless of size. In California, ADUs under 800 sq ft are exempt from impact fees, so the 400 sq ft footprint is in the most fee-favorable range.
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