How much does a 1,000 sq ft ADU cost to build?
A 1,000-square-foot ADU costs $100,000 to $300,000 to build, per HomeGuide — its $100 to $300 per square foot across 1,000 feet.
The band is three times as wide at the top as at the bottom, and that is not vagueness. On HomeGuide's own tables the construction type moves the number more than anything else does: a prefab modular unit is $80 to $160 per square foot, an attached unit $125 to $225, a detached one $150 to $250, and one built over an existing garage $200 to $500 or more. At 1,000 square feet those are four genuinely different projects wearing one label.
Source: HomeGuide
This page is general information, not professional advice. Remodeling prices move with your local labor market, the condition of what is already there and what your building department requires, and the only number that counts is a written quote from a contractor who has walked the job.
The same 1,000 feet, priced by construction type
| Type | Per square foot | 1,000 sq ft |
|---|---|---|
| Basement conversion | $50 – $100+ | $50,000 – $100,000+ |
| Garage conversion | $50 – $200 | $50,000 – $200,000 |
| Prefab modular | $80 – $160 | $80,000 – $160,000 |
| Attached | $125 – $225 | $125,000 – $225,000 |
| Detached | $150 – $250 | $150,000 – $250,000 |
| Above garage | $200 – $500+ | $200,000 – $500,000+ |
Source: HomeGuide
That table is the published per-square-foot band carried across 1,000 feet — our arithmetic on HomeGuide's figures, shown so it can be checked. HomeGuide's own by-size table stops at 1,200 square feet and reads $100,000 to $300,000 at 1,000 and $120,000 to $360,000 at 1,200, which is the same $100 to $300 rate. Both readings agree because both are the rate.
Source: HomeGuide
Why 1,000 square feet is a different conversation from 600
Because at that size the cheap routes start running out. HomeGuide's garage conversion band assumes an existing garage, and a 1,000-square-foot garage is a three-car or four-car structure most lots do not have. Its basement conversion band assumes a basement of that footprint already under the house. What is left for a 1,000-square-foot unit on most properties is attached or detached construction, and those are the two most expensive rows on its table that are not built on a roof.
Source: HomeGuide
Size is the input everyone types. Construction type is the input that actually sets the price. Decide the second one before you argue about the first.
What sits outside the per-square-foot number
HomeGuide publishes the soft costs separately: house plans at $1,200 to $5,000, a land survey at $400 to $1,800, site excavation at $1,500 to $5,000, grading and levelling at $1,000 to $5,000, a foundation at $2,400 to $10,500 and building permits at $500 to $2,000. It also puts labor at 40% to 60% of the whole project, and says a custom build uses more professional labor than a prefab because it needs designers, an architect or engineer, a general contractor, an electrician and a plumber rather than a factory shell and a crew to set it.
Source: HomeGuide
For comparison, HomeGuide's home addition guide prices an ADU addition at $100 to $300 per square foot — the same band — and a plain room addition at $125 to $250, which says something worth knowing: at 1,000 square feet a full ADU with its own kitchen and bathroom is priced in the same territory as an ordinary addition of the same size, and the kitchen and bath are what push it toward the top of the band rather than off it.
A 1,000-square-foot ADU is a small house, and the bid spread on it is enormous. These are the general contractors AI assistants actually name when homeowners ask. Nobody can pay to be on these lists.
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Questions people ask next
- Is a 1,000 sq ft ADU cheaper per square foot than a small one?
- Not on HomeGuide's tables. HomeGuide's band is $100 to $300 per square foot at every size from 400 to 1,200 square feet, so the total scales with the area rather than falling with it. What changes the rate is the construction type, not the footprint.
- How much does a 1,200 sq ft ADU cost?
- HomeGuide publishes $120,000 to $360,000 for a 1,200-square-foot ADU, the largest row on its by-size table, at the same $100 to $300 per square foot.
- Can a 1,000 sq ft ADU be prefab?
- HomeGuide prices prefab modular at $80 to $160 per square foot, which reads $80,000 to $160,000 across 1,000 square feet, and says prefab is cheaper because a factory does not lose days to weather, material deliveries or crew scheduling. Whether a unit that size can be delivered to your lot is a site-access question for the builder.
- Does a 1,000 sq ft ADU add value to the home?
- HomeGuide says a detached ADU will in many cases increase a home's value, that the effect is larger where housing demand is high, and that a well-built unit with modern amenities is more likely to help than a low-quality one. It publishes no percentage, so neither does this page.
Written for homeowners by Most Recommended Contractors, which measures which general contractors AI assistants actually recommend in 26 U.S. metros. Nobody can pay to be on those lists. How we measure.