ADU Cost Calculator - Cost to Build an ADU by Type (2026)

An ADU costs $100 to $500 per square foot in 2026, or roughly $100,000 to $400,000 for a finished unit. A garage or basement conversion runs $100-$280/sf, an attached ADU $180-$420/sf, and a detached new build $250-$500/sf. The big jump comes from building a new foundation, roof, and utility connections from scratch instead of reusing an existing structure.

A 600 sq ft detached ADU costs $155,000-$237,000 at standard finish on a new slab. As a garage conversion, the same 600 sq ft runs $60,000-$120,000 because you reuse the foundation and roof. Premium finishes - custom cabinets, tile, hardwood - push a 600 sq ft detached unit past $300,000.

Yes. A garage conversion costs 40-60% less than a comparable detached ADU because the foundation, walls, and roof already exist. Your money goes into insulation, egress windows, plumbing, electrical, and finishes instead of a whole new structure. The trade-off is you lose the garage, and older garage slabs sometimes need reinforcement or a moisture barrier to pass inspection as living space.

A permitted ADU typically returns 60-100% of its cost at resale and generates rental income in the meantime. A detached, legally permitted unit adds the most value because it functions as a separate rentable dwelling. As with any addition, prices vary by region - get local quotes and confirm the unit is permitted, because appraisers discount or exclude unpermitted square footage.

I price ADUs by the square foot for the shell, then add foundation, utility connections, and permits as separate line items because those fixed costs are where quotes go wrong. For a detached unit I start at $250-$380/sf, add $8-$15/sf for the slab, then layer in $10,000-$40,000 for sewer, water, and electrical trenching plus permit and impact fees. Use our Contractor Estimate Template to turn those line items into a clean bid the homeowner can actually read.

An ADU estimate takes me 2 to 4 hours done by hand because it is essentially a small house - foundation, framing, a full kitchen, a bath, and separate utility connections all have to be priced. The slow part is the utility tie-in and confirming what the city requires. With EstimationPro I cut that to under an hour - I walk the site, talk through the scope, and it builds the line-item estimate so I can quote the homeowner in days instead of weeks.

An ADU almost always requires a building permit, plus electrical, plumbing, and mechanical permits, and often a planning or zoning review. Permit and impact fees run $3,000-$25,000 depending on your city - some jurisdictions waive impact fees for ADUs to encourage housing, others charge full price. Never skip permits. An unpermitted ADU cannot be legally rented and gets excluded from your home's square footage at resale.

The cheapest ADU is a garage or basement conversion at $100-$280/sq ft, because you reuse the existing foundation, walls, and roof. A prefab or modular detached unit is the next most affordable path for a stand-alone build. To keep any ADU affordable, reuse existing utility runs where possible, stick to a simple rectangular footprint, and choose builder-grade finishes. Get multiple bids from local contractors before you commit.