How Much Does a 1 Bedroom ADU Cost?

A one bedroom ADU runs $39,900–$225,000 all in, nationally, at a standard finish. That is a wide band, and it is wide for an honest reason: bedroom count is not a unit of measurement.

So here is the assumption this whole page rests on, stated up front. I price a one bedroom ADU at 400 to 600 square feet, which is the band our own Los Angeles and Washington cost pages already use when they talk about layouts. Under 400 you are describing a studio. Over 600 you are describing a one bedroom with a lot of living room, and the numbers on our 800 square foot page get closer.

A detached one bedroom prices $120,000 at 400 sq ft and $180,000 at 600 sq ft. Hold on to that $60,000 difference, because it is about to lose an argument.

1 bedroom ADU cost by type

Every build type across the 400 to 600 square foot band, at a standard finish, nationally. This is the only page in the silo where all five types are genuinely available, and that is worth understanding before you read the numbers.

The junior ADU row is priced at its statutory ceiling of 500 square feet rather than across the full band, because it cannot legally go higher. At $59,850 it is less than half the cost of anything else on this table, and it is the reason the bottom of the headline range is so low.

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 One Bedroom

Your finish level matters more than your square footage does. Moving from 400 sq ft to 600 sq ft costs $60,000 on a detached build. Moving from a budget finish to a high-end one at the middle of that band costs $63,000. Those come out of the same engine and they are almost the same number, with the finish decision slightly ahead. I have watched people agonise over a hundred square feet and then pick cabinets in ten minutes, and it is the wrong way round.

This is the last size where a junior ADU is legal. California defines a JADU as no more than 500 square feet of interior livable space, contained entirely within a single-family residence. A 400 or 500 square foot one bedroom can be a JADU. A 600 square foot one cannot. If you have a spare bedroom near existing plumbing, read the junior ADU guide before you price a detached build, because the cheapest one bedroom on this page is a room you already own.

You are comfortably inside every California threshold. The entire 400 to 600 square foot band sits under the 750 square foot impact fee line in Government Code section 66311.5(c)(1), so no impact fees apply anywhere in the state. It also sits well under the 850 square foot minimum maximum in section 66321(b)(2), so no city ordinance can tell you a one bedroom ADU is too big.

That combination is why I push first-time builders toward this size. Not because it is cheap, though it is the cheapest real home on the site, but because nothing about it is contested.

If you want to see what this size actually looks like laid out, we have 6 original 1 bedroom floor plans drawn to scale with room dimensions, each with a walkthrough of what works and what to watch.

1 bedroom ADU cost by city

A one bedroom ADU priced through every city index we maintain, at the middle of the 400 to 600 square foot band. The full range column spans the whole band and every build type, which is why it is so wide.

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

The finish spread at the middle of the one bedroom band, and the table that makes the point above. $63,000 between the bottom and the top, on a unit whose entire square footage question is worth $60,000.

National point estimates at 500 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 $150,000 goes on a 500 sq ft detached one bedroom at a standard finish, nationally.

Detached ADU, 500 sq ft, standard finish, national. $300 per sq ft all in, with a planning range of $112,500–$187,500.

Frequently asked questions

How Much Does a 1 Bedroom ADU Cost?

Nationally, $39,900–$225,000 all in at a standard finish, priced across 400 to 600 square feet and every build type. A detached one bedroom runs $120,000 to $180,000 depending on where in that band it lands, and a junior ADU carved from existing space is far cheaper. California prices roughly $273,000 for a detached unit at the middle of the band.

How Many Square Feet Is a 1 Bedroom ADU?

Typically 400 to 600 square feet, which is the assumption every number on this page uses. Below 400 you are really pricing a studio, where the bedroom is not separated. Above 600 the extra space goes into the living area rather than the bedroom, so the cost climbs without the layout changing much.

Can a 1 Bedroom ADU Be a Junior ADU?

Yes, if it is 500 square feet or less and sits entirely within the walls of the existing single-family residence. Government Code section 66313 caps a junior ADU at 500 square feet of interior livable space, so a 400 or 500 square foot one bedroom qualifies and a 600 square foot one does not. A JADU is the cheapest legal one bedroom there is, at the cost of a recorded deed restriction and, where sanitation is shared, an owner-occupancy requirement.

Does a 1 Bedroom ADU Pay Impact Fees in California?

No. The whole 400 to 600 square foot band is under the 750 square foot threshold in Government Code section 66311.5(c)(1), which prohibits any impact fee on an ADU of that size or smaller. Building permit and plan check fees are a separate matter and still apply.

Is a 1 Bedroom or a 2 Bedroom ADU a Better Investment?

A one bedroom usually wins on cost per rented square foot and a two bedroom usually wins on total rent. The second bedroom on our two bedroom page costs real money in construction and, in California, moves you above the 750 square foot fee threshold. If the unit is for family, build for the family. If it is for yield, run both numbers against local rents before you decide.

If I were building a one bedroom today, I would build 600 sq ft at a budget finish rather than 400 sq ft at a high-end one. Same money, roughly, and space is the thing a tenant cannot upgrade later.

The exception is a spare bedroom sitting next to a bathroom wall. In that case build the junior ADU, spend a third of the money, and go and read what a two bedroom costs when you are ready for the real project.