How Much Does an ADU Cost? A 2026 Price Guide
Quick answer: Most ADUs cost between $100,000 and $300,000 to build in 2026. A detached new build runs higher, around $150,000 to $300,000 or more. A garage conversion is usually cheaper, around $80,000 to $150,000. Price depends on size, type, finish level, and where you live. Figure roughly $200 to $500 per square foot.
An ADU, or accessory dwelling unit, is a second living space on a property where someone already lives. People call them granny flats, backyard cottages, casitas, in law suites, or garage apartments. Whatever you call it, the first question every homeowner asks is the same. What is this going to cost me?
This guide breaks it down by type, by size, and by the things that move the price up or down. Real ranges, no fluff.
ADU cost at a glance
Here are typical all in ranges for 2026. All in means design, permits, and construction together.
These are national ranges. Your actual cost depends a lot on your city, your lot, and the finishes you pick. A high cost market like California or a tight infill lot can push you well past these numbers.
What drives the price
A handful of things decide where you land in those ranges.
Size
Bigger costs more, but not in a straight line. The kitchen and bathroom are the most expensive rooms per square foot. A 400 square foot studio and an 800 square foot one bedroom both need one kitchen and one bath, so the smaller unit often costs more per square foot than you would expect.
Type
A garage conversion reuses an existing structure, so you save on the foundation, framing, and roof. That is why it is the cheapest path. A detached new build starts from dirt, so it costs the most.
Site work
This is the hidden one. Running water, sewer, and power to a detached unit in the backyard can add tens of thousands of dollars, especially if the lines have to cross the whole property. A flat, easy to access lot saves money. A sloped or tight lot costs more.
Finishes
Builder grade finishes keep you at the low end. Quartz counters, custom cabinets, nice flooring, and high end fixtures push you up fast. This is the part you control most.
Where you live
Labor and material costs swing hard by region. The same ADU can cost 40 percent more in one metro than another. Permit fees vary too, sometimes by a lot.
ADU cost by type, explained
Garage conversion ADU
The budget friendly option. You are working inside walls that already exist, so the big structural costs are already paid for. The work is mostly insulation, drywall, plumbing, electrical, a kitchen, and a bathroom. Most garage conversions land between $80,000 and $150,000. See our full breakdown of garage to ADU conversion cost.
Attached ADU
This is an addition that shares a wall with the main house. It costs more than a garage conversion because you are building new square footage, but less than a fully detached unit because you share a wall and sometimes utilities. Expect $100,000 to $200,000.
Detached ADU
A standalone unit in the backyard. The most private, the most flexible, and the most expensive. You are building a small house from the ground up, with its own foundation, roof, and utility connections. Most detached ADUs run $150,000 to $300,000 or more.
Prefab and modular ADU
Built in a factory and dropped on your lot. Can save time and sometimes money, but site prep, foundation, and utility hookups still cost what they cost. Total often lands similar to a site built unit once everything is connected.
What about permits
Permit costs vary widely by city, from a few hundred dollars to several thousand, and impact fees in some areas add more. Budget a few thousand to be safe and check your local rules. We cover this in detail on our ADU permit cost guide.
Is an ADU worth it
For a lot of homeowners, yes. An ADU can add rental income, house family, and raise the value of your property. The key is going in with a real budget and a builder who gives you a clear, itemized quote instead of a vague number.
The smartest move is to get quotes from two or three local ADU builders who can walk your lot and price your specific project. National ranges are a starting point. Your lot, your city, and your finishes set the real number.
Frequently asked questions
How much does it cost to build an ADU?
Most ADUs cost between $100,000 and $300,000 in 2026. A garage conversion can be done for $80,000 to $150,000, while a detached new build often runs $150,000 to $300,000 or more, depending on size, finish, and location.
What is the cheapest type of ADU to build?
A garage conversion is usually the cheapest because it reuses an existing structure, saving on foundation, framing, and roofing. Most fall between $80,000 and $150,000.
How much does an ADU cost per square foot?
Most ADUs run $200 to $500 per square foot all in. Smaller units often cost more per square foot because the kitchen and bath are fixed costs spread over less space. See our ADU cost per square foot guide.
Do ADUs add value to a home?
Yes, in most markets an ADU adds value through added square footage and the potential for rental income. The exact value depends on your local housing market.
How long does it take to build an ADU?
A garage conversion can take a couple of months. A detached new build usually takes four to eight months from permit to finish, depending on your city and the builder's schedule.
Building ADUs for a living?
Quick note for the contractors reading this. If you build ADUs and you are tired of riding the referral roller coaster, the homeowners on this page are the exact people we put in front of builders every day. See how ADU marketing for builders works.