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What is an Accessory Dwelling Unit (ADU)?
An ADU is a second, smaller dwelling unit located on the same lot as a primary residential building. ADUs are intended as a housing option that can contribute to housing affordability and choice, respond to changing household needs and lifestyles, contribute to housing supply, and contribute to neighborhood stability consistent with residential neighborhood patterns. ADUs can be located either within or attached to the primary structure, or in a detached structure in the rear yard.
Where ADUs are allowed?
One ADU per lot is allowed as an accessory use to a primary residential structure, on legally established lots (irrespective of lot size or width) in zoning districts that permit residential development.
An attached ADU (Dalton home, www.accessorydwellings.org)
A Detached ADU (Photo by Lauren Flemister, City of Tacoma)
Development Standards
Size
The living area of the ADU, excluding any non-living areas, is limited to the most restrictive of the following:
- No more than 85% of the living area of the primary structure or dwelling.
- Detached ADUs may be no more than 85% of the footprint of the main structure.
- No more than 1,000 square feet for standard lots or 750 square feet for small lots.
- Attached ADUs located entirely on a single floor of an existing building can use the entire floor area.
Height
Attached ADUs are subject to the primary structure height limits.
Detached ADUs shall be no taller than the most restrictive of the following:
- Up to 18 feet total height (if measured to the midpoint of a roof, a height survey will be required).
- Up to 20 feet total height with incorporation of parking, or with Built Green 4 stars (or equivalent) environmental certification (if measured to the midpoint of a roof, a height survey will be required).
- Conversion of an existing accessory structure taller than 18 feet may be considered as a Conditional Use.
- In View Sensitive Districts, maximum height is 15 feet (measured per TMC 13.01.060, additional height is subject to TMC 1305.010.B variance criteria).
Setbacks & location
Attached ADUs are subject to the primary structure setbacks.
Detached ADUs must:
- Be on the same lot as the primary structure.
- Be in the rear yard of the primary structure.
- Provide a minimum of 5 feet from side and rear property lines, except no setback is required from alleys.
- Meet Building Code fire separation from other structures.
- Meet accessory building standards (TMC 13.06.020).
Parking
Parking is not required for ADUs. If parking is proposed, it must be located in and accessed from the rear portion of the lot. The addition of an ADU may not remove parking required for the primary dwelling.
Open Space
The overall site must maintain or provide at least 10% of the lot size as usable and functional yard space (per TMC 13.06.020).
Walkways
For ADUs with a separate entrance, a walkway must connect to the nearest public sidewalk or right-of-way.
- A minimum 3-foot walkway width is required.
- Walkway materials must be distinct from any adjacent vehicular areas (may function as a shared drive/walk space along an exterior edge of a driving surface).
Design Standards for all ADUs
- Generally match the exterior architectural style, appearance and character of the main house with similar materials, window, façade and roof design; or complement the main house through use of materials and design of equal or better quality.
- Meet applicable Historic District design standards.
Design Standards for Detached ADUs
- Main entrances at least 8 feet from side property lines shared with a neighbor, if the entrance faces the neighboring property.
- Second story windows within 10 feet of shared property lines shall be constructed to reduce direct views into the neighboring property.
- The structure shall not intercept a 45-degree daylight plane inclined into the site from a height of 15 feet above existing grade from the 5-foot setback line.
Other Requirements
- Both dwellings must be in common ownership.
- ADUs shall include facilities for cooking, living, sanitation, and sleeping.
- A separate address visible from the street is required.
- Occupancy and minimum room dimensions shall meet Tacoma Municipal Code 2.01.060 sections S and T.
- Construction code standards include minimum ceiling heights, minimum insulation, WSEC energy credits, fire separation, bedrooms emergency escape, smoke and carbon monoxide alarms.
- Each dwelling needs access to maintenance and control their own heating and water heater.
- Attached ADU’s need interconnected smoke and carbon monoxide alarms.
- Utility services are required per City standards and need external service connections to sewer and water outside each detached unit.
- Standards for home occupations, short-term and long-term rentals apply.
See Tacoma’s ADU Design Guide for further information, illustrations, departmental contacts and permit submittal guidance.
Permit Requirements
Construction permits are required for creating, converting, or legalizing an ADU. The type of permits are dependent on the scope of the project, and could include: Building, Plumbing, Mechanical, Site Development, Side Sewer, Stormwater, Water and/or Electrical.
Applicable Regulations
Tacoma Municipal Code (TMC) Section: 13.06 Land Use Code, TMC Title 2 Building and Plumbing Code, TMC Title 12 Utilities.
Submittal and Review Process
Application materials must be submitted in electronic PDF format online at TacomaPermits.org. See tip sheet on Electronic File Standards G-230.
Submittal requirements include plans drawn to scale with adequate information to understand the new construction and/or modifications meeting design requirements and applicable codes for construction and inspection. A clear scope of work also assists. More information can be found at Accessory Dwelling Unit Help Page
Permit Application and Review
- Apply online at www.tacomapermits.org or directly through the Permitting Portal (ACA)
- Plan review times can be seen at this link .
After work is completed, contact PDS to request a new address and schedule necessary inspections.
Example Attached and Detached ADU site plans
