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What to Know about building ADUs in Seattle

Whether you’re looking to build an ADU (Accessory Dwelling Unit) for a family member, for rental income, or as an addition to your home, there are a few key things you should know before getting started. We’ll cover the basics of ADUs in Seattle, from zoning regulations to construction requirements. We’ll also provide some helpful tips for making the process as smooth as possible.

So, if you’re thinking of building an ADU in the Emerald City, read on!

Why are ADUs becoming more popular in Seattle?

The popularity of accessory dwelling units (ADUs) is on the rise across the West Coast. In the Seattle area, there was a 250% ADU construction increase in 2022 compared to 2019. Given their smaller size and lack of additional land cost, ADUs can offer a more affordable housing option in neighborhoods where homes are out of reach to most people.

What are the new ADU rules in Seattle?

The new ADU rules in Seattle were signed into legislation by Mayor Jenny Durkan in July 2019. These regulations aim to remove regulatory barriers and make it easier for property owners to create accessory dwelling units (ADUs) in Seattle’s neighborhood residential zones. The new rules took effect on August 8, 2019. For more detailed information about the new legislation, you can refer to the following resources:

  • Legislative history of the Accessory Dwelling Unit Ordinance
  • Mayor’s Executive Order 2019-04: Actions to encourage more affordable accessory dwelling units throughout Seattle
  • Office of Planning and Community Development webpage: Encouraging Backyard Cottages

Zoning and permitting requirements
The specific requirements depend on the zoning of the property. Here are some key points:

  1. Zoning: ADUs can be built in neighborhood residential (NR) zones, lowrise zones, and some neighborhood or commercial zones. However, there are limitations on the size and placement of ADUs in different zones.
  2. Number of ADUs: In NR3, NR2, and NR1 zones, up to two ADUs can be constructed. This can be either two attached accessory dwelling units (AADUs) or one AADU and one detached accessory dwelling unit (DADU). In Neighborhood Residential Small Lots (RSL) and multifamily lowrise zones, only one attached or detached ADU is permitted for each single-family, rowhouse, or townhouse unit.
  3. Size Limitations: AADUs are limited to 1,000 square feet in neighborhood residential zones, including RSL, and up to 650 square feet in lowrise zones. DADUs are also limited to 1,000 square feet in neighborhood residential zones, including RSL, and 650 square feet in lowrise zones.

Tips for planning and building an ADU

Hire a qualified contractor

A contractor experienced with ADU projects will be knowledgeable about the local building codes, zoning regulations, and necessary permits. They can provide valuable guidance throughout the design, planning, and construction process, helping to avoid costly mistakes and delays.

Get all the necessary permits

Before starting any construction, it’s vital to obtain all necessary permits and approvals to avoid potential legal complications down the line. In Seattle, this includes building, electrical, mechanical, and plumbing permits. You may also need to get clearance from the Environmental Critical Areas (ECAs), depending on the location of the project.


Building an ADU in Seattle is an interesting investment, given its increasing popularity and the city’s supportive regulations. However, the process requires thorough planning and careful execution. Understanding the local zoning and construction requirements, hiring an experienced contractor, and getting all the necessary permits are key steps. By doing so, you’ll ensure that your ADU project is executed smoothly and successfully. Remember, the goal is not just to build an ADU but to create a comfortable and affordable living space that meets your specific needs and contributes to the overall housing solution in Seattle. Good luck on your ADU journey!

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