How much does it cost to build an ADU?
An ADU costs $100 to $300 per square foot to build, or $60,000 to $180,000 for a 600-square-foot unit, per HomeGuide.
An accessory dwelling unit is a self-contained second home on the same lot as the main house, with its own kitchen and its own bathroom — HomeGuide's definition, and the one that separates an ADU from a guest house, which is built for short stays and needs neither. That kitchen-and-bath requirement is why the per-square-foot band starts where it does: an ADU is a small house, not a finished room.
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, the finishes you pick and what your building department requires, and the only number that counts is a written quote from a contractor who has walked the job.
ADU cost by size
| ADU size | Average cost |
|---|---|
| 400 sq ft | $40,000 – $120,000 |
| 500 sq ft | $50,000 – $150,000 |
| 600 sq ft | $60,000 – $180,000 |
| 750 sq ft | $75,000 – $225,000 |
| 1,000 sq ft | $100,000 – $300,000 |
| 1,200 sq ft | $120,000 – $360,000 |
Source: HomeGuide
The table is the rate times the area, which is the honest way to read it: HomeGuide's $100 to $300 per square foot carried straight across every row. That is also why the spread is so wide. A 1,000-square-foot ADU is $100,000 at the bottom of the band and $300,000 at the top of it on the same page, and which end you land on is decided by the construction type below, not by the square footage.
Source: HomeGuide
ADU cost per square foot, by construction type
| Type | Per square foot | What it is |
|---|---|---|
| Basement conversion | $50 – $100+ | Finishing or excavating basement space under the main house |
| Garage conversion | $50 – $200 | Repurposing an existing garage into habitable space |
| Prefab modular | $80 – $160 | Built in a factory, assembled on site |
| Attached | $125 – $225 | Joined to the main house by a shared wall or extension |
| Detached | $150 – $250 | A freestanding structure, usually in the backyard |
| Above garage | $200 – $500+ | A new second storey built over an existing garage |
Source: HomeGuide
HomeGuide reads the same 600-square-foot unit at $50,000 to $100,000 prefab, $75,000 to $135,000 attached, $90,000 to $150,000 detached and $120,000 to $300,000 above a garage. Its reasoning for the ordering is plain: an attached unit shares the existing heating, plumbing and electrical, a detached one has to have all three run out to it, and building over a garage means adding a storey to a structure that was never designed to carry one. A garage conversion is the cheapest route at $20,000 to $140,000 for a full conversion, and HomeGuide adds the caveat that it can lower the home's value where garage parking is scarce.
Source: HomeGuide
The cheapest ADU is almost always the one that reuses a foundation and a roof you already own. The expensive part of a backyard cottage is that it has neither.
What you spend before anyone frames a wall
| Line | Average cost |
|---|---|
| House plans | $1,200 – $5,000 |
| Land survey | $400 – $1,800 |
| Site excavation | $1,500 – $5,000 |
| Grading and levelling | $1,000 – $5,000 |
| Foundation | $2,400 – $10,500 |
| Building permits | $500 – $2,000 |
Source: HomeGuide
HomeGuide puts labor at 40% to 60% of the whole project and says a custom build uses more of it than a prefab — designers, an architect or engineer, a general contractor, an electrician and a plumber against a factory shell and a contractor to set it. The first question to settle is not the price. It is whether your city allows an ADU on your lot at all: HomeGuide's own guidance is to check the zoning before spending anything on drawings.
Source: HomeGuide
To price a size and a construction type against the published tables: the home addition cost calculator.
Three written quotes on the same size, the same construction type and the same finish level is the comparison every published guide asks for. 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
- How much does a 1,000 sq ft ADU cost to build?
- HomeGuide publishes $100,000 to $300,000 for a 1,000-square-foot ADU, which is its $100 to $300 per square foot carried across 1,000 feet. Where inside that band you land is set by the construction type: HomeGuide's detached band is $150 to $250 per square foot and its prefab band is $80 to $160.
- What is the difference between an ADU and a guest house?
- HomeGuide draws it on the kitchen and the bathroom. An ADU is built for permanent, full-time occupancy and must have both. A guest house is for short stays, needs neither because the visitors use the main house, and HomeGuide prices a custom one at $150 to $250 per square foot or a modular one at $80 to $160.
- Is a prefab ADU cheaper?
- On HomeGuide's table, yes — $80 to $160 per square foot against $150 to $250 for a detached custom build, or about $50,000 to $100,000 for a 600-square-foot unit. Its explanation is that a factory does not lose days to weather, late material deliveries or crew scheduling.
- Do I need a permit to build an ADU?
- HomeGuide lists building permits at $500 to $2,000 and says permitting cost varies by location and unit size. It also says outright that some cities do not allow ADUs at all, so the zoning check comes before the budget.
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.