ADU Costs in Seattle

Three ADU Build Types

A garage or basement conversion is typically the most affordable ADU path — existing shell and utilities reduce cost, though retrofitting for living use still requires substantial work. An attached ADU adds a new conditioned space to the main home and tends to fall in the mid range.

A detached new-construction ADU carries the highest cost because it requires a full foundation, shell, utility lateral, and a separate permit track. These projects are essentially small homes and are priced accordingly.

Utility Lateral Work Is a Common Surprise

Connecting an ADU to water, sewer, electrical, and sometimes gas is often one of the largest single cost lines — sometimes more than expected. Some lots can't add a second sewer lateral without major work, which can shift the budget substantially. Confirming utility feasibility early is one of the most important savings moves an ADU project can make.

Foundation and Site Work

For detached ADUs, foundation and site work are significant: excavation, footings, drainage, and the connection of utilities all happen before framing starts. Sloped lots, poor drainage, and tight access all amplify site costs.

Design for the Purpose

A rental ADU calls for durable finishes and easy-to-maintain systems; a family ADU can prioritize comfort and aging-in-place. These goals aren't the same, and finish selections, layout, and mechanical choices should reflect which one you're building for — the right choices also keep cost aligned with the intended use.