How Much Does a 400 Sq Ft ADU Cost?

A 400 square foot ADU runs $39,900–$150,000 all in, nationally, at a standard finish. The bottom is a junior ADU carved out of your house at $53,200; a detached unit lands around $120,000. In California the same detached build prices around $218,400, and our California ADU cost breakdown has the regional detail.

Four hundred square feet is the smallest ADU I would build for someone to live in full time. It fits a studio comfortably and a one bedroom only just, and it is the size where the per square foot price climbs fastest, which is the part most cost pages leave out.

400 sq ft ADU cost by type

All five build types work at 400 square feet, which is rare. The spread between the cheapest and the dearest is $66,800, and almost all of it is foundation and site work: a junior ADU and a garage conversion reuse structure you already own.

National, standard finish, before any city adjustment. Typical all-in is the point estimate; the range is the engine's residual spread for site conditions, utilities and contractor.

What Changes at 400 Square Feet

Cost per square foot goes up, and the table above understates it. Our engine holds the per square foot rate flat across sizes, so it prices a 400 square foot detached unit at $120,000, or $300 a foot. But 27% of a detached budget is a kitchen and bath, the utility connections, permits and design, and those lines barely shrink with the unit. Hold them at what they cost on an 800 square foot build and the same 400 square feet comes to $152,400, or $381 a foot. Real bids land somewhere between the two.

Studio or one bedroom? At 400 square feet I draw studios. A separate bedroom is possible, but it takes the space out of the living room. Here are our 400 square foot layouts side by side, from the drawings themselves:

The smallest one bedroom we draw is a 440 square foot two-car garage conversion, which our 1 bedroom ADU floor plans include. Our studio ADU floor plans show both 400 square foot layouts drawn to scale.

You are well under the California impact fee line. Government Code section 66311.5(c)(1) bars impact fees on an ADU of 750 square feet or less of interior livable space. Permit, plan check and utility connection charges still apply.

A junior ADU is on the table. California caps a JADU at 500 square feet of interior livable space inside the house, so a 400 square foot unit carved from existing rooms qualifies. Our junior ADU guide covers the owner-occupancy and kitchen rules.

Prefab is worth pricing at this size. Our California cost guide puts a 400 square foot prefab unit at $145,000–$230,000 installed. My breakdown of what a prefab ADU costs installed splits that into the factory unit and the site work, and shows where the saving shrinks on small units.

If you want to see what this size actually looks like laid out, we have 5 original 400 sq ft floor plans drawn to scale with room dimensions, each with a walkthrough of what works and what to watch.

400 sq ft ADU cost by city

The same 400 square feet through every city cost index we maintain, at a standard finish.

Standard finish. The cost index is that city's published average against our national reference, so these columns move whenever a city cost page does. Full range spans every eligible build type at this size.

What finish level does to it

Finish level moves a 400 square foot detached budget by $50,400 from bottom to top. On a unit this small, the kitchen and the bath are most of what a finish upgrade buys.

National point estimates at 400 sq ft. The junior ADU column, where it appears, is priced at its 500 sq ft statutory cap rather than at this page's size.

A worked budget

Here is where $120,000 goes on a 400 square foot detached build at a standard finish, nationally, split across the engine's detached cost profile. Remember the caveat above: on a unit this small the fixed lines run higher than this flat split shows.

Detached ADU, 400 sq ft, standard finish, national. $300 per sq ft all in, with a planning range of $90,000–$150,000.

A budget is only half the question. For how homeowners actually cover a number like this, with the honest math on home equity, renovation and construction loans and the 2026 rental-income rules, see our guide to financing an ADU.

Frequently asked questions

How Much Is a 400 Sq Ft ADU?

Nationally, $39,900–$150,000 all in at a standard finish: about $53,200 as a junior ADU, $73,200 as a garage conversion and $120,000 as a detached unit, per our cost engine. In California a detached 400 square foot unit prices around $218,400.

Can Two People Live in 400 Square Feet?

Yes, as a couple, in a well-planned studio. The plans that work give the bed a wall of its own, put the kitchen on one run and keep the bath door out of the living space. Two adults who are not a couple will want a one bedroom at 440 square feet or more.

Is a 400 Sq Ft ADU a Tiny Home?

By the building code definition, roughly yes. IRC Appendix Q defines a tiny house as a dwelling of 400 square feet or less, excluding lofts. But a tiny house on a foundation is still permitted as an ADU, and one on wheels is usually treated as an RV. Our tiny house in the backyard guide covers the difference.

Why Does a Small ADU Cost More per Square Foot?

Because a kitchen, a bathroom, the utility connections and the permit are needed at any size, and they do not get much cheaper when the unit shrinks. Those lines are about 27% of a detached budget, which is why a 400 square foot unit can run closer to $381 a foot than the $300 a flat rate suggests.

If I were building 400 square feet today, I would build a studio, spend the money on the kitchen, the bath and one big window, and skip the separate bedroom. Then I would price what 500 square feet costs, because the next hundred feet are the cheapest you will ever buy on an ADU.