How Much Does a 600 Sq Ft ADU Cost?

A 600 square foot ADU runs $82,350–$225,000 all in, nationally, at a standard finish. The bottom of that band is a garage conversion at $109,800, the top is a detached unit at $180,000, and the gap between those two is bigger than most people expect at this size.

In California, where most of the people asking this question live, the same detached build prices around $327,600. Our California ADU cost breakdown has the state-level context, and the city table further down this page prices 600 square feet in every market we cover.

600 is my favourite number on this whole silo, and it is not because of the construction cost. It is because of where it sits relative to two lines that other people draw for you: the 750 square foot fee threshold above it, and the 500 square foot junior ADU cap below it.

600 sq ft ADU cost by type

Same 600 square feet, five different ways to build it, and a $70,200 spread between the cheapest and the dearest. This is the table I wish more people looked at before they decided what they were building.

The reason the spread is so wide is structural, not cosmetic. A garage conversion adu reuses a slab, a roof and three walls you already own, so its $183 per square foot base rate is doing less work than the detached $300. At 600 square feet that difference is worth $70,200.

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 600 Square Feet

You are under the California impact fee line, and it is worth real money. Government Code section 66311.5(c)(1), renumbered and amended by SB 543 effective January 1, 2026, prohibits a local agency, special district or water corporation from imposing any impact fee on an ADU of 750 square feet or less of interior livable space. Not reduced. None.

What that is worth depends on the city, and the honest answer is that most published fee schedules make it hard to see. Campbell's does not. Campbell's park in-lieu fee is $0 under 750 sq ft and jumps to $9,328 at 750 sq ft or larger. That single line is the difference between roughly $7,700 and $17,100 in combined city fees, so it is a design decision rather than a surprise bill. City of Campbell FY2026-27 Master Fee Schedule, effective July 1, 2026. Our Campbell cost page walks the rest of that fee stack.

One thing to watch, because I have now seen it twice at a planning counter: Saratoga's own published ADU handout still waives impact fees under 700 sq ft, but SB 543 sets the floor at 750 sq ft of interior livable space statewide as of January 1, 2026. State law wins, so the city's handout understates your exemption by 50 sq ft. City of Saratoga ADU standards handout, August 2024, read against SB 543 (2025 to 2026 session) If a handout and the statute disagree, the statute is the one that governs.

A junior ADU is off the table here. California defines a JADU as no more than 500 square feet of interior livable space contained entirely within a single-family residence, so 600 square feet is past the cap by definition rather than by ordinance. If the cheap end of this page is what drew you in, read the junior ADU guide and decide whether you would rather have 500 square feet with no foundation than 600 with one.

It is the last size where a garage conversion is an ordinary project. Our garage conversion page publishes a typical band of 300 to 650 square feet, which is roughly a two-car garage. At 600 you are inside that. At 800 you are not, and the price on this page stops meaning what it says.

If you want to see what this size actually looks like laid out, we have 3 original 600 sq ft floor plans drawn to scale with room dimensions, each with a walkthrough of what works and what to watch.

600 sq ft ADU cost by city

Same 600 square feet, same standard finish, priced through every city cost index we maintain. The index is the ratio of that city's published average to our national reference, so these columns move when the city pages move.

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 600 square foot budget by $75,600 from bottom to top. That is more than the entire cost of the kitchen and bath line in the worked budget below, which is the part that surprises people.

National point estimates at 600 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 $180,000 actually goes on a 600 square foot detached build at a standard finish, nationally. Every line is the engine splitting that total across the detached cost profile, so the parts sum to the whole exactly.

Detached ADU, 600 sq ft, standard finish, national. $300 per sq ft all in, with a planning range of $135,000–$225,000.

Frequently asked questions

How Much Does a 600 Sq Ft ADU Cost?

Nationally, $82,350–$225,000 all in at a standard finish, depending entirely on which of the five build types you pick. A garage conversion lands around $109,800 and a detached unit around $180,000. In California the same detached build prices around $327,600, and in the Bay Area cities on the table above it runs higher still.

Is 600 Sq Ft Big Enough for a One Bedroom ADU?

Yes, comfortably, and it is the size I recommend most often for a first ADU. 600 square feet takes a real bedroom, a full bath, a proper kitchen and a living area without the layout compromises you make at 400. If you want a second bedroom you are looking at 700 sq ft and up, which our two bedroom cost page prices out.

Do I Pay Impact Fees on a 600 Sq Ft ADU in California?

No. Government Code section 66311.5(c)(1) prohibits any impact fee on an ADU of 750 square feet or less of interior livable space, so a 600 square foot unit is exempt statewide as of January 1, 2026. Building permit fees, plan check fees and utility connection charges are separate and still apply, so read your city's fee schedule rather than assuming the whole permit package is free.

Can a 600 Sq Ft ADU Be a Junior ADU?

No. A junior ADU is capped at 500 square feet of interior livable space under Government Code section 66313, so 600 square feet has to be a full ADU. That is usually the better outcome anyway: a full ADU carries no owner-occupancy question and no recorded deed restriction, which the junior ADU guide covers in detail.

What Is the Cheapest Way to Build 600 Square Feet?

A garage conversion, at roughly $109,800 against $180,000 for a detached build, which is a $70,200 difference on identical square footage. The catch is that it only works if the existing garage passes on ceiling height, slab condition, egress and distance to the sewer. When one of those fails, the saving evaporates fast.

If I were building 600 square feet today and I had a sound two-car garage, I would convert it and put the $70,200 I saved into the kitchen and the windows, where a tenant actually feels it.

If I did not have that garage, I would price the detached build at 600 rather than at 800, precisely because of the fee line. Then I would look at what 800 square feet costs and decide whether the extra room is worth losing the exemption.