ADU Cost to Build (2026)

By the Home Building Cost Calculator team · Updated November 2026

An ADU (accessory dwelling unit) costs about $180,000 on average to build in 2026, with most projects between $40,000 and $360,000, or $150 to $300 per sq ft. Converting a garage or basement is the cheapest route at $60,000 to $150,000; a new detached backyard cottage costs the most.

How much does an ADU cost by type?

ADU typeTypical cost
Garage or basement conversion$60,000–$150,000
Attached ADU (addition)$100,000–$216,000
Detached ADU (backyard cottage)$110,000–$285,000
Above-garage ADU$128,000–$225,000
All ADUs$40,000–$360,000, average $180,000

How much does an ADU cost by size?

SizeAt $150/sq ftAt $300/sq ft
400 sq ft$60,000$120,000
600 sq ft$90,000$180,000
800 sq ft$120,000$240,000
1,000 sq ft$150,000$300,000

What adds to the cost of an ADU?

  • Design and engineering: about 10% to 15% of the project.
  • Permits: around $1,350 on average, plus impact or utility fees in some cities.
  • Utilities: extending water, sewer and power from the main house, or upgrading the electrical panel.
  • Site access: tight backyards can mean hand-carrying materials and higher labor costs.
  • Kitchen and bathroom: the most expensive rooms per square foot, and an ADU needs both.

Why do ADUs cost more per square foot than a house?

An ADU is small, but it needs a full kitchen, a bathroom, its own heating and cooling, utility connections and the same permits and inspections as a house. Those fixed costs are spread over fewer square feet. Our calculator models a detached ADU at about 1.35 times the cost per sq ft of a standard house in the same state.

Frequently asked questions

Does an ADU add value to a house?

It can, through rental income or added living space, but the gain depends on local rents, zoning and buyer demand.

Can I finance an ADU?

Common options include a home equity loan or line of credit, a cash-out refinance, a renovation loan or a construction loan.

Compare other house types

Sources