ADU Cost Calculator

Adding an accessory dwelling unit, whether you convert a garage, build a detached structure from scratch, or finish a basement, is a major capital project, and a solid cost estimate is what makes financing and budgeting possible. An early read on the numbers helps you sidestep nasty surprises and weigh the investment with clear eyes.

The estimate multiplies the square footage you enter (200 to 1,200 sq ft) by a cost-per-square-foot range for the type of unit: $125-$200 for a garage conversion, $200-$350 for a detached new build and $100-$175 for a basement. The midpoint of that range is the construction cost, and a permit allowance of 5% of it is added to reach the project total. The ranges are fixed, with no adjustment for your region or finish level and no contingency, so set aside your own cushion for the unforeseen. The annual ROI shown is gross yield: twelve months of the rent you enter divided by the construction midpoint, before vacancy, upkeep, taxes or financing.

Permit costs swing widely from one municipality to the next, so confirm your local fees rather than assuming. For detached units in particular, site work is the line item people tend to lowball, since excavation and utility trenching can run deep and expensive. Be equally cautious of bids that come in suspiciously low, they often leave out finishes or system upgrades that you will end up paying for anyway.