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. With construction material prices projected to settle in 2026 following a stretch of volatility, an early read on the numbers helps you sidestep nasty surprises and weigh the investment with clear eyes.

The estimate runs on a multi-factor method anchored to average 2026 construction costs per square foot, then tuned for regional differences and the complexity of your particular build. It accounts for base construction, the major systems of plumbing, electrical, and HVAC, interior finishes, and soft costs such as permits and architectural design, blending those into a weighted average across the different ADU types. A 5-15% contingency is folded in for the unforeseen, which is essential to any honest budget.

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.