Accessory Dwelling Units

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What is an accessory dwelling unit?
Otherwise known as a “mother-in-law apartment”, an ADU is – by definition of the zoning code – a complete, independent residence with provisions for cooking, living, sanitation, and sleeping that is a second unit, accessory to the primary single-family residence on the property. ADUs are allowed in certain areas of the City to add affordable units to the existing housing supply, provide an increased choice of housing that responds to changing needs, lifestyles (e.g., young families, retired), and increase density to better utilize existing infrastructure and community resources (i.e. support public transit and neighborhood businesses).

Example of Attached ADU

Example of Detached ADU

Basic Requirements
To qualify for an ADU, the minimum development standards below will need to be met. You (property owner on title) must also live on the property, either in the main dwelling or the ADU. A separate Building Permit or Minimum Housing Code Inspection may be required. Driveways, utilities, and paving/gravel on the property may need separate permits.

  • Location
    • ADUs are allowed in all areas, except the industrial districts of M-2 and PMI.
    • In the residential districts of R-1, R-2, R-2SRD and HMR-SRD Districts, only attached ADUs are allowed.
  • Minimum lot size does not apply to attached ADUs that fit within the building envelope of an existing single-family detached dwelling. All other attached ADUs must meet the minimum Level 1 lot size for single-family detached dwellings in the applicable zoning district.
    • Detached ADUs must meet the minimum standard lot area and lot width of the zoning district.
  • ADU size must be from 200 – 1,000 square feet. Floor area maximum is also equal to 40% of the main home and ADU square footage combined, not including garage space
    • Detached ADUs also need to meet standards in TMC 13.06.100.F.
  • Setbacks for attached ADUs are the same as the main dwelling.
    • Detached ADUs must be located in the rear yard and have a minimum 5-foot setback from property lines, except along an alley.
  • Parking is not required for the ADU. If parking is proposed, it must be located and accessed from the side or rear of the lot if possible.
  • Design must include facilities for cooking, living, sanitation, and sleeping.
    • The design for detached ADUs must be consistent with the architectural style, appearance, and character of the main home.
    • Attached ADUs must maintain the appearance of a single-family residence.
    • Utilize complimentary colors and finish materials, window styles, and roof design.
    • For ADUs with a separate entrance, provide a 4-ft wide pedestrian path from the ADU to the sidewalk or street that is made of different materials than any surrounding vehicle areas
  • Garage Conversion An existing garage may be converted into an ADU if the garage is upgraded to meet all of the applicable Building and Energy Codes, and if it meets the zoning standards for detached or attached ADUs.
  • Height Attached ADUs have the same height limit as the main home. Detached ADUs are limited to 18 feet in height and shall be no taller than the main house (see TMC 13.06.150.D.3 for specifics).  A height survey may be required with your application.
  • Occupancy A maximum of 4 people may live in an ADU per the zoning code. The City’s minimum housing code standards also limit the number of residents to prevent overcrowding.

Submittal Checklist
All application materials must be submitted in electronic PDF format on compact disc (CD) or online at

See tip sheet on Electronic File Standards G-230.

  • Application for Land Use Permit, signed and dated
  • Site plans, floor plans and building elevations to scale
    • (It is helpful to provide a layout of the existing floor plan as well as the proposed floor plan.)
  • Other plans necessary to obtain permits associated with the creating a habitable space may include building, plumbing, mechanical, & electrical permits.
  • If a new sewer connection or a connection to the on-site side sewer is proposed, a Side Sewer Connection Permit is also needed.
  • Permit Fee

**Note: The creation of an ADU may trigger additional
improvements to meet building code, such as the
installation of a firewall or an additional egress

Review Process

  1. Submit a Land Use Application. If applicable, also submit building permit application or minimum housing inspection application. You can apply in person at Planning and Development Services (PDS), Tacoma Municipal Building, 747 Market Street, 3rd Floor. Or you can apply online at The plans may need a 1-3 week review time.
  2. After the review is complete, you can record a Notice on Title at the Pierce County Assessor’s Office and provide a copy of the recorded document to PDS.
  3. PDS will issue a Letter of Approval for the ADU.
  4. After work is completed, you will contact PDS to request a new address for the ADU and schedule Final Building Inspection.

Example of an attached ADU site plan


Example of a detached ADU site plan


Applicable Regulations