Investment Strategy · FAQ

How much does a detached ADU cost to build in Seattle?

Last updated: August 2026

Quick Answer

Typically $200,000 to $350,000 for a well-finished 600–1,000 square foot unit, depending on size, finish level, and site conditions. Attached ADUs (basement or garage conversions) generally cost less, in the $80,000 to $180,000 range.

From the article

ADUs and Investment Properties in Seattle: A 2026 Owner's Guide

Washington's statewide zoning reform has made ADUs one of the most accessible ways to add rental income or multigenerational space to a Seattle property. Here's what it actually costs, what's allowed under 2026 rules, and how ADUs fit into a broader investment strategy.

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People also ask

How many ADUs can I build on my Seattle lot in 2026?

Most single-family lots can accommodate up to two ADUs — one attached and one detached — under current Seattle and Washington State rules, though specific lot size, setback, and critical-area restrictions can affect this. A pre-construction consultation confirms your specific property's capacity.

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Do I have to live on the property to build an ADU?

No. The owner-occupancy requirement that previously applied in Seattle was removed, meaning owners can build and rent out an ADU without living on-site themselves.

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Is building an ADU a good investment compared to buying a rental property?

For many owners, yes — an ADU avoids the acquisition cost, financing, and search process of a separate property, and offers flexibility (family housing now, rental income later) that a standalone investment property doesn't. The trade-off is that the ADU's value is tied to your primary property rather than being a separately sellable asset.

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