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New 后院的小屋 (D/ADU) regulations now in effect in Seattle!

master suite addition ADU with outdoor living craftsman home seattle

The City of Seattle wants you to builds an ADU!

This week marks a big change in how Seattle allows backyard cottages, also known as Mother-in-Law units or Granny Suites. The names for these Accessory Dwelling Units (ADUs) are Attached Accessory Dwelling Unit (AADU) for apartments either inside or attached to the primary house on a residential lot, or 分离 Accessory Dwelling Unit (大都) for separate structures.  Seattle has allowed these secondary dwelling units in our residential neighborhoods for many years, but in an effort to increase housing options, has loosened the regulations affecting their design. 截至8月8日th 2019, these updated regulations are now in effect!  In fact, now just about every single family lot can have TWO accessory dwelling units!


We’re getting lots of calls from home owners who are curious and a little confused about what they can or cannot build within these new rules. This blog will outline the new 大都 and AADU regulations, and give a primer on how to think about getting started on your own 大都 or AADU project!


一开始, if you’re not familiar with zoning lingo, you may not know that your lot falls into a zone category that dictates the type of 建筑 allowed on it. If your lot has a single house on it, and maybe a separate garage (or not), it’s most likely your zoning type is Single Family 5000 (SF5000) or similar. 也就是说每块地都有一栋房子, and includes a set of restrictions like maximum lot coverage (35%+/- ), 需要挫折, 最大允许高度, one parking spot required in limited areas on the lot, 等.  I’m going to limit this discussion to SF lots, since most of our residential work is within this zoning category.



  • In SF5000, SF7200, and SF9600 zones, up to two ADUs may be constructed. This can be either two AADUs (Attached ADU) or one AADU and one 大都 (detached).
  • The minimum lot size required for a 大都 is 3,200 square feet in single-family zones, 和25英尺的地段宽度.
  • Each AADU or 大都 can be no larger than 1,000sf of living space. You can build an additional 40 sf of unheated space for bike storage. 如果你的大都大学有车库, the garage sq footage is not included in the 1,000年科幻最大计算.
  • Maximum height limit for a 大都 ranges between 14 to 18ft, 取决于你的地段宽度, plus additional height for a sloped roof.
  • adu不需要停车. 然而, you cannot remove existing required off-street parking space(s) unless replaced elsewhere on the property in a location that is allowed under the code. Every single family lot is required to maintain 1 legally sited parking spot (if there is an existing spot on that property).
  • The property owner is not required to live on the property where an ADU(s) is located.
  • The D/ADU must meet current standards of the Seattle residential, 建筑, 机械, 电, 能源, 土地使用, 环境关键领域, 和海岸线代码


Other things to think about that have not changed, but impact your planning for a D/ADU:

  • Maximum lot coverage remains at 35% of the lot size. 如果你有后院小巷, you can include ½ the width of that alley for your overall lot size in your lot coverage calculations!
  • Setback requirements remain the same. These vary depending on your lot size and configuration, but know that a 大都 can be built in your rear yard setback, 一次又一次, 如果你回到巷子里, the centerline of the alley may allow you a little more room. Instead of only allowing 40% of your required rear yard (setback) to be covered with accessory structures (ie garage and/or 大都), you can now cover up to 60% of a required rear yard with your accessory structure.
  • ADU的成本:它很贵! 这通常是最引人注目的. New construction costs about $300/sf or more, which means it’s pretty hard to build a new 大都 for under $300k. Of course the cost will vary depending on the structure’s size, 具有挑战性的网站工作, and type of contractor you work with. Attached ADU’s within an existing house (ie converting a basement into an apartment) will generally be far less costly than a 分离 ADU.


这是很多信息, 但总体来说, the new rules are much less restrictive and will certainly enable more and bigger ADUs to be built. ADUs are a pretty great idea; we have clients who build them for older parents to be able to live in close proximity to the younger family with grandchildren, or for visiting family and then rent the space on AirBnB when vacant, or for long-term rental enabling a cash stream – helping with mortgage payments!


请随意 打电话给betway中文版 any time for more information if you’re considering 建筑 a second (or third!)住宅单位在你的物业上. We’ve designed and built many of these structures, so can give you quick and reliable feedback on your particular situation.