FAQ – Seattle ADUs & Backyard Cottages

Thinking of building an ADU or Backyard Cottage (aka Detached ADU) in Seattle? Great! Check out our “ADU 101” and “How Much Does It Cost To Build An ADU?” posts as you’re getting started.

You’ll also want to peruse one of these “Tips” publications from the City of Seattle: “Establishing An Attached Accessory Dwelling Unit,” “Accessory Dwelling Unit (Mother-in-Law Apartment)” or “Establishing A Backyard Cottage.” From these publications (as of May 2015), we’ve distilled a few answers to frequently asked questions:

Q: Does my main house or ADU/Backyard Cottage need to be owner occupied?

A: Yes. For at least six months of the year, either the main house or the ADU/Backyard Cottage need to be your main and permanent residence.

Q: How big can my ADU or Backyard Cottage be?

A: Attached ADUs can be 1000 square feet or smaller (with a couple exceptions) for single family homes. For rowhouses and townhomes, Attached ADUs can be no larger than 650 square feet and may not exceed 40% of gross floor area. Backyard Cottages can be 800 square feet or smaller, including garage and storage areas.

Q: Do I have to provide parking for my ADU or Backyard Cottage?

A: Yes, one off-street parking space is required, except in some urban villages and urban centers, or in lowrise zones.

Q: How large does my lot need to be to allow a Backyard Cottage?

A: At least 4000 square feet.

Q: How much does it cost to build an ADU or Backyard Cottage in Seattle?

A: See our post, “How Much Does It Cost To Build An ADU?

Q: What else should I consider?

A: See our post, “ADU 101.

Good luck with your project. Please contact us if you’d like us to build it!

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