This ADU cost calculator estimates the full cost of building an accessory dwelling unit (also called a granny flat, backyard cottage or in-law suite), including design, permits, utilities and a contingency. It then works out the loan payment, rental cash flow, yield and payback period.
What goes into ADU cost
- Construction: size × cost per square foot. Garage conversions usually cost the least per square foot and detached new builds the most.
- Soft costs: architect or design, structural engineering, surveys and permit fees.
- Site work and utilities: water, sewer and electrical connections, trenching and foundations.
- Contingency: 10% or more for surprises.
Worked example
A 600 sq ft ADU at $300 per sq ft is $180,000 to build. With a 10% contingency, $20,000 of soft costs and $15,000 for utilities, the total is $233,000. Renting it for $2,000 a month with 5% vacancy and $350 of monthly costs gives about $18,600 a year, a payback of about 12.5 years before any rise in property value.
Ways to pay for an ADU
- Cash or savings.
- A home equity loan or HELOC.
- A cash-out refinance.
- Renovation or construction loans, some of which lend against the expected value after building.
Check local rules first
Zoning, size limits, setbacks, parking, impact fees and whether you must live on the property all vary by city and county. Many US states, Canadian provinces and Australian states have eased rules for secondary dwellings, but local approval is still needed.
Frequently asked questions
How much does it cost to build an ADU?
It varies widely by region, design and site. Get several local quotes per square foot and add design, permit, utility and contingency costs, which the calculator includes.
Does an ADU add value to my home?
Often, because it adds living space and rental income potential. How much depends on your local market and how buyers value the unit.
Can I rent out my ADU?
Usually, but some areas restrict short-term rentals or require the owner to live on the property.
How long does it take to build an ADU?
Design and permits often take several months, and construction several more. Prefabricated units can be faster.
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Last reviewed: October 2026. For information only; not financial, tax or legal advice. Results are estimates.