Accessory Dwelling Units

Detached adu in Olympia neighborhood

What is an ADU?

Commonly referred to as a "mother-in-law apartment" or "granny flat", an Accessory Dwelling Unit (ADU) is an additional living unit with separate kitchen, sleeping area and bathroom facilities.

ADUs can be attached to or detached from the primary residence on a single-family lot. They can be attached to the home or garage or built as a small, stand-alone, guest house style structure.

Community Advantages

  • ADUs provide flexible and affordable housing options.
  • ADUs provide more low-impact and private rental housing within single-family neighborhoods.
  • ADUs promote infill development (development of largely vacant and undeveloped land within areas that are already largely developed) to help preserve the surrounding natural greenbelt.

Homeowner Advantages

  • ADUs offer housing for extended family who want to remain close, but need independence.
  • ADUs provide an alternative for senior homeowners who want to age in place without the expense and maintenance of a larger home.
  • ADUs can supplement homeowner income with a low maintenance rental unit.
  • ADUs may increase property value.

Renter Advantages

  • ADUs provide affordable housing opportunities in single-family neighborhoods otherwise unaffordable to some.
  • ADUs are an option for people who want a small, private living unit but don't want to live in an apartment complex.

What Type of ADU is Right for You?

There is a limit of one ADU per property, and only associated with a single family home. There are benefits and challenges associated with each type.


  • Building a new structure gives you more flexibility in design and location within your property, and potentially more privacy from the tenants of the unit.


  • There may be more fees and more material needed to construct a new unit, potentially making it a more costly option.


  • Could be a cost benefit option depending on the quality of the garage structure that you are converting. You do not need to take additional space from your property. The “bones” of the unit are ready and only inside work may be needed.


  • It can be challenging to bring a space to full compliance with the building code. (Ex: wall thickness may differ from garage to habitable space, insulation of concrete floors, adding an emergency egress (escape) from the sleeping area). The ADU will also need a fire resistant wall. There may be additional costs for bringing water/sewer pipes, electricity, and heat to the unit if it is detached from the main house.
  • The property will need to maintain required onsite (off-street) parking spaces (typically 2 spaces for the main house and 1 for the ADU). The driveway or other paved area may work for some, but if the garage is the only onsite parking, it could be a challenge to provide parking elsewhere onsite.


  • When building above your garage, you are not using any of your land. If the second floor is an additional construction, you can build it up to code and have flexibility in the design. It can save you the costs of laying a new foundation.


  • There is a height limit of 16 feet for ADUs, which may be challenge when building above a garage. This rule may not apply if the garage is attached to a two-story home. The ADU cannot exceed 800 square feet, including garage.  If your garage is larger than 400 square feet, this may pose a challenge.  Exceeding the size or height limit will require a "Conditional Use Permit" adding time and cost to your project.


  • In this scenario, you would have a space in your home that you do not use, and is already habitable. This could be a cost saving option depending on where your space is situated and how much work is needed to turn it into an ADU. You would not need to take any space away from your property.


  • It may be a challenge to add a separate entry and have privacy between the main house space and the ADU space. There will be plumbing work necessary to add another bathroom and a kitchen area to a new space of the home.

Initial Questions and Considerations

Keep these important things in mind when planning to build or convert an existing space into an ADU.

You must obtain a permit from the City of Olympia before you begin construction! Permitting costs vary depending on the value of the ADU. A permitting specialist will evaluate the permitting costs and any fees associated with your project.

Whether you are building a new structure or remodeling part of your home or garage, your project will be subject to design reviews and inspections at various times to make sure it is being built according to the approved plans and that you are in compliance with building, plumbing, mechanical and electrical codes.

The ADU must have a separate entry and preserve the privacy of the main house occupants and neighbors.

An ADU can be used to accommodate a family, or become a rental unit. A rental unit creates a potential income, and the owner must conform to landlord regulations, decide on rent, and how utilities will be billed.  The homeowner must live on the premises and a covenant must be filed with the Thurston County Auditor's office prior to receiving a Certificate of Occupancy from the City.

 One of the requirements for ADUs in that the homeowner must provide an additional 8' x 16' parking space for the tenant in addition to the 2 parking spaces required for the main house.

Although you have a choice, it is much cheaper to extend sewer and water to the ADU from the primary home, rather than the street.

The Building Code establishes minimum requirements for habitable structures. The ADU must not exceed 800 square feet, and the City recommends the minimum size not to be below 200 square feet.

The ADU must be between 190 and 800 square feet. There must be at least one habitable room with a minimum gross floor area of 120 square feet (11m2) and a minimum length of 7 feet. Other rooms must be at least 70 square feet (6.5m2). If a room has a sloping ceiling, any part of that room with a ceiling below 5 feet in height is not considered as habitable.

The detached ADU cannot exceed 16 feet in height, including when added above a garage.

Detached garages over 800 square feet, with or without an ADU, require a Conditional Use Permit (ask staff for details).

There must be a separate entry for the ADU.

The ADU must match the character of the main house.

See “Zoning and Design standards” for more requirements.

It is recommended to enroll the help of professionals to design the ADU to ensure it will follow health and safety regulations. Hiring help to build it may add costs to the addition. While some homeowners are able to build it themselves, others may need help. The choice can alter the initial costs as well as the quality of the addition.

The style of the new structure must match that of the house, in order to preserve the neighborhood’s character.

ADU Design and Construction

The Building Code establishes minimum requirements for habitable structures. The maximum size allowed for an ADU is 800 square feet and the minimum recommended size is 200 square feet.

  • Every dwelling unit shall have at least one habitable room that shall have not less than 120 square feet (11m2) of gross floor area.
  • Other rooms (except kitchen) shall have a floor area of not less than 70 square feet (6.5m2).
  • Habitable rooms (except kitchen) shall not be less than 7 feet (2134 mm) in any horizontal dimension.
  • Portions of a room with a sloping ceiling measuring less than 5 feet (1524 mm) or a furred ceiling measuring less than 7 feet (2134 mm) from the finished floor to the ceiling shall not be considered as contributing to the minimum required habitable area for that room.


Contact the Permitting Center Monday - Friday, 8 a.m. - 4 p.m.

In Person: Olympia City Hall - 2nd Floor, 601 4th Avenue E
By Phone: 360.753.8314
Via Email: