In July 2019, new legislation was signed into law making it easier to build detached and attached accessory dwelling units in Seattle (ADU’s and DADU’s). While Seattle was one of the earlier major metropolitan areas to embrace ADU’s and DADU’s we did so in a typical Seattle fashion and saddled them with a series of restrictions that ensured that not many were built. We were soon outpaced in number of units built by many other cities allowing these types of structures as their laws were less restrictive than ours. Recent code changes have started us on the path to rectifying that.

Before we talk about the changes and what they might mean, we should acknowledge who got us here. A huge thank you is owed to councilmember O’Brien who originated the legislation and the advocates and supporters such as Matt Hutchins, Sheri Newbold and the rest of MOAR.

Now… What has changed. There are quite a few changes, but we have found that the most important to our clients are the following:

Reduces the minimum lot size required to build a DADU on a single-family lot from 4,000 square feet to 3,200 square feet. - This is going to mean more options for DADUs across Seattle. 

Increase the maximum size of DADUs from 800 square feet to 1,000 square feet. - This added square footage is going to make DADU’s more feasible for small families.

Parking areas are not counted toward the size of the DADU if within the same structure: - If you are planning on converting a part of a garage to a DADU, this is huge.

Removes the owner-occupancy requirement for ADUs. - This will make it easier for DADUs to add to the much needed housing market in Seattle

Removes the off-street parking requirement for ADUs. - Not having to eat up valuable lot space for parking is a game changer. 

Allows two ADUs on one lot (either one attached and one detached, or two attached) if the second ADU meets a green building standard or will be affordable to households at or below 80% of area median income. - This is great as you can have an ADU within your house and a DADU in the yard. This gets really fun if you work with your neighbor and create a mini cottage housing project with six units on two lots.

Increases DADU height limits a bit. This will help tremendously as the previous limits made ceiling heights quite tight for two story cottages.

Note that the above text is a summary of some of the changes. For the full text of the legislation, see (

Now that it is a little easier to build a DADU, what can we do with them?

Seattle backyard cottage
Seattle Backyard Cottage
Garage backyard cottage

And for the fun stuff..... We updated our DADU cost post to reflect more current data. Take a look!

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