How Much Does a 1,200 Sq Ft ADU Cost?

A 1,200 square foot ADU runs $164,700–$450,000 all in, nationally, at a standard finish. Detached, which at this size is the only type I would plan against seriously, prices around $360,000. In California, closer to $655,200.

That detached figure is the top of the range our detached ADU guide publishes, and it is not a coincidence. 1,200 square feet is where an accessory dwelling unit stops being accessory to anything and becomes a small house in your back yard.

Which is fine. It is also the size where the budget is the second question, because in California nothing obliges your city to let you build it.

1,200 sq ft ADU cost by type

The five types at 1,200 square feet. Three of these rows are priced past what the type page itself calls typical, and I would not budget from them without a conversation about what the project actually is.

Which leaves detached at $360,000 as the real answer. At 1,200 square feet you are building a two bedroom house with a foundation, a roof and a full mechanical system, and there is no clever type choice that changes that.

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 1,200 Square Feet

You are past every state size floor, so the ordinance decides. Government Code section 66321(b)(2) stops a California city from capping an ADU below 850 square feet, or below 1,000 square feet where the unit provides more than one bedroom. It says nothing about 1,200. Above 1,000 you are asking permission rather than exercising a right, and the answer varies city by city.

That single fact reorders the whole project. On every other page in this silo you can budget first and confirm later. Here you confirm your local maximum ADU size first, because a beautiful 1,200 square foot budget is worth nothing if the counter caps you at 1,000.

Impact fees are fully in play, and proportionally larger. Section 66311.5(c)(2) charges fees on anything over 750 square feet proportionately in relation to the square footage of the primary dwelling. At 1,200 square feet against a 2,000 square foot house that is 60 percent of the fee, and against a smaller primary dwelling it can approach the whole thing.

Compare the per square foot rate to your own house. At $300 per square foot all in nationally, a 1,200 square foot detached ADU costs about what a modest new build costs, without the land. That is the honest frame for this size. It is also why financing tends to shift from a home equity line toward construction financing right around here, which our ADU financing breakdown covers.

If you want to see what this size actually looks like laid out, we have 2 original 1,200 sq ft floor plans drawn to scale with room dimensions, each with a walkthrough of what works and what to watch.

1,200 sq ft ADU cost by city

1,200 square feet through every city index we maintain. Read this table knowing that in several of these cities the local size cap will stop you before the price does.

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 is worth $151,200 at 1,200 square feet. At this size I would spend it on the envelope and the windows rather than the cabinets, because you are heating and cooling a real house now.

National point estimates at 1,200 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

Where $360,000 goes on a 1,200 square foot detached build at a standard finish, nationally.

Detached ADU, 1,200 sq ft, standard finish, national. $300 per sq ft all in, with a planning range of $270,000–$450,000.

Frequently asked questions

How Much Does a 1,200 Sq Ft ADU Cost?

Nationally, $164,700–$450,000 all in at a standard finish across build types, with a detached unit around $360,000. In California the same build prices near $655,200. At this size the practical answer is the detached figure, because the conversion types are well past what they are typically used for.

Is a 1,200 Sq Ft ADU Even Allowed?

It depends on your city. California's Government Code section 66321(b)(2) only requires local ordinances to allow up to 850 square feet, or 1,000 square feet where the ADU has more than one bedroom. Anything above 1,000 is a local decision, so confirm your jurisdiction's maximum ADU size before you spend money on plans at this size.

What Is the Cost Per Square Foot of a 1,200 Sq Ft ADU?

About $300 per square foot all in nationally at a standard finish for a detached build, before any city adjustment. Our model holds the rate flat across sizes on purpose rather than modelling a discount for scale, so treat the per square foot figure as a planning rate and expect real bids to soften slightly at larger footprints.

Should I Build 1,200 Sq Ft or Two Smaller Units?

In California you can often have one ADU and one junior ADU on a single-family lot, which is a genuinely different strategy from one large unit. Two smaller units usually rent for more in total and hedge your vacancy risk, at the cost of a second kitchen and a second bath. The junior ADU guide covers how that pairing works.

If I were building 1,200 square feet, my first call would be to the planning counter and my first question would be the maximum ADU size in the ordinance, not the fee schedule. Everything else on this page is downstream of that answer.

If the answer is 1,000, do not fight it. Build the 1,000 square foot version, which the state requires them to allow for a multi-bedroom unit, and put the $60,000 you saved somewhere it earns rent.