Accessory Dwelling Unit Laws in Washington State


Like in many other states throughout America, accessory dwelling unit (ADU) laws differ greatly on a city and county basis. Hence, below is an overview of some of the tiny house laws in some of Washington State’s most populous localities. They include:

  • Seattle
  • Kent 
  • Vancouver, WA

Seattle

The city of Seattle has the requirements listed relating to ADUs in the Seattle Municipal Code which can be found here. A specific table that is present in the code details requirements for a detached ADU in  Seattle. Below we have highlighted some of these specifics from the table:

Minimum lot size 3,200 square feet
Minimum lot width 25 feet
Front yard A detached accessory dwelling unit may not be located within the front yard required by subsection 23.44.014.B, except on a through lot pursuant to Section 23.40.030 or Section 23.40.035.
Maximum size The gross floor area of a detached accessory dwelling unit may not exceed 1,000 square feet excluding garage and storage areas, porches and covered decks that are less than 25 square feet in area, and gross floor that is underground. Up to 35 square feet of floor area dedicated to long-term bicycle parking shall be exempt from the gross floor area calculation for a detached accessory dwelling unit. The bicycle parking area shall be provided in a safe, and convenient location, emphasizing user convenience and theft deterrence, and shall be located where bicyclists are not required to carry bicycles on stairs to access the parking. Where practicable, long-term bicycle parking shall include a variety of rack types to accommodate different types of bicycles.

An extract from the code also states that under certain circumstances ‘in an SF 5000, SF 7200, or SF 9600 zone, a lot with or proposed for a principal single-family dwelling unit may have up to two accessory dwelling units

Kent 


The Kent City Code details the regulations surrounding accessory dwelling units in the city. They can be found here

An extract from the Code is below that details some of the requirements around constructing an ADU in the city:

  • 1. One ADU per detached single-family dwelling unit is allowed within all residential zones. There shall be not more than one guest cottage or ADU on any one lot.
  • 2. An ADU may be established within or as an addition to a new or existing single-family dwelling, or as a detached unit from the principal dwelling. All ADUs not established within or as an addition to the principal dwelling shall be deemed “detached,” regardless of whether they are attached to any accessory structure or building on the lot. Detached ADUs are subject to the provisions of KCC 15.08.160.
  • 3. The ADU, as well as the principal dwelling unit, must meet all applicable setbacks, lot coverage, and building height requirements.
  • 4. The design and size of an ADU shall conform to the requirements of all building, plumbing, electrical, mechanical, fire, health, and any other applicable codes. When there are practical difficulties involved in carrying out the provisions of this section, the building official may grant modifications for individual cases pursuant to the International Building Code, the International Residential Code, or other applicable building codes, as subsequently amended or re-codified.
  • 5. One of the dwelling units shall be owner-occupied as the owner’s principal residence for at least six months every calendar year. No permit for an ADU will be issued until the owner files a covenant evidencing this use limitation against the property. This covenant must be recorded in the King County recorder’s office, at the owner’s expense, and shall be in a form acceptable to the city attorney.
  • 6. If either the ADU or the principal unit ceases to be owner-occupied for at least six months in any given calendar year, the ADU permit shall be deemed revoked and use of the unit as an ADU must cease immediately.
  • 7. The size of an ADU contained within or attached to an existing single-family dwelling shall be limited by the existing dwelling’s applicable zoning requirements. An ADU incorporated into the construction of a new single-family dwelling shall be limited to 40 percent of the principal dwelling unit.
  • 8. The size of a detached ADU, for either new construction or an existing home, shall be up to 800 square feet or 33 percent of the size of the principal unit, whichever is smaller. If the detached ADU is built within, above, or in addition to another accessory structure, the footprint of the accessory structure is not counted towards the footprint of the ADU.

Vancouver, Wa

There is a document on the city’s website that details the Accessory Dwelling Unit rules. This document can be found here.  An extract from the code that details the specifics regarding constructing an ADU is below:

A. Configuration. An ADU may be located either within, attached to, or detached from the primary structure. 

B. Density. Only one ADU may be created in conjunction with each single-family residence. 

C. Minimum lot size. An ADU may be established on any legally established parcel meeting applicable standards of this chapter. 

D. Maximum unit size. The gross floor area, calculated from finished wall to finished wall. ADU shall not exceed 800 square feet or 50 % of the primary single-family structure, not including garage and/or detached accessory buildings (whichever is less). ADUs created entirely within existing basements may exceed 800 square feet provided they are not larger than the size of the remainder of the overall home. 

E. Minimum unit size. The gross floor area of an ADU shall not be less than the requirements of the Washington State Building Code.

F. Setbacks and lot coverage. Additions to existing structures, or the construction of new detached structures, associated with the establishment of an ADU shall not exceed the allowable lot coverage or encroach into required setbacks as prescribed in the underlying zone. The applicable setbacks shall be the same as those prescribed for the primary structure, not those prescribed for detached accessory structures. 

G. Scale and visual subordination. New detached ADUs, or ADUs extending from existing structures shall not comprise more than 50% of total visible façade area of the primary structure and other outbuildings not including the ADU, as seen from the front of the lot. ADUs shall be subject to a maximum height of 25 feet. 

H. Parking. No additional on-site parking is required in conjunction with the establishment of an ADU. 

I. Design and appearance. ADUs that are separate or extending from existing structures shall be architecturally compatible with the principal dwelling.

J. Construction standards. The design and construction of the ADU shall conform to all applicable standards in the building, plumbing, electrical, mechanical, fire, health and any other applicable codes.’

Please make sure to contact the city directly to see if any changes have been made to ADU rules.

Conclusion on Washington State ADU laws

Localities throughout Washington have different laws relating to accessory dwelling units. In Seatlle, under certain circumstances, a lot with or proposed for a principal single-family dwelling unit may have up to two accessory dwelling units in an SF 5000, SF 7200, or SF 9600 zone. However, in Vancouver WA, only one ADU may be created in conjunction with each single-family residence. 

To find out more about ADU laws in areas that weren’t mentioned above, make sure to contact the relevant city and county in question directly.

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This article is written for informational purposes only and is not to be construed as legal advice.

Sources

  • City of Seattle, The Seattle Municipal Code § 23.44.041 (2020)
  • City of Kent, Kent City Code § 15.08.350 (2020)
  • cityofvancouver.us, Accessory Dwelling Units § 20.810.040

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