Washington ADU Laws and Permits: A Comprehensive Resource


Here are the most frequently asked questions about building ADUs in Olympia.

  1. What are the size limitations for an ADU?

    An ADU must have a gross floor area of no more than 850 square feet.

  2. Can an ADU be attached to other structures like garages?

    Yes, ADUs may be attached to accessory structures such as garages or shops, and these structures can be up to the maximum allowed sizes, generally 1200 square feet for detached garages.

  3. What are the setback requirements for an ADU?

    ADUs must be set back five feet from any interior side property line and ten feet from a flanking street side yard. If the rear yard doesn’t abut an alley, the setback must be ten feet from the rear property line.

  4. Are there any specific building height restrictions for ADUs?

    Yes, the maximum height for detached ADUs is 24 feet, measured at mid-gable.

  5. Do ADUs require fire sprinklers?

    ADUs must have fire sprinklers if the existing home is sprinklered or if building codes require sprinklers for the existing home.

  6. What design criteria must ADUs meet?

    ADUs should reflect the architectural character of the primary residence, using similar roof forms, window patterns, facade materials, and colors.

  7. Is a parking space required for an ADU?

    A parking space is allowed but not mandatory for ADUs. Some areas may require a residential parking pass for street parking.

  8. What fees are associated with building an ADU?

    Impact fees are applicable and are paid at the time of building permit issuance.

  9. What is the process for design review of an ADU?

    The design review is part of the building permit application process, ensuring that the ADU complies with local architectural standards.

  10. Can the city sewer and water connections be shared with the primary residence?

    Yes, connections to city sewer and water services can share the same connections as the primary home.

  11. What types of ADUs are allowed?

    Detached, attached, or internal ADUs are permitted.

  12. Can I build an ADU on a lot with a single-family home?

    Yes, one ADU is allowed per lot in conjunction with a detached single-family home.

  13. Are there specific requirements for ADUs in new subdivisions?

    Yes, refer to OMC 18.04.080(A)(3) for specifics on ADUs in new subdivisions.

  14. How should the ADU be oriented in relation to neighboring dwellings?

    The ADU should be oriented to maintain privacy for residents in adjoining dwellings as much as possible.

  15. What are the occupancy limits for an ADU?

    No more than one family is allowed to occupy an ADU.

  16. Can deviations from standard ADU requirements be granted?

    Yes, deviations can be allowed by the Director for efficient use of space or for ADUs in older structures.

  17. What are the conditions for allowing deviations in ADU construction?

    Conditions include the age of the building and location in relation to property setbacks.

  18. What are the rules for ADUs in structures built before June 19, 1995?

    These structures can house ADUs if they meet building codes and development standards, even if located in setbacks.

  19. How are covered porches or patios treated in ADU floor area calculations?

    They do not count toward the gross floor area but are limited to 120 square feet and may not be enclosed.

  20. Are there any specific design review criteria for ADUs?

    ADUs must undergo a design review to ensure they match the architectural character of the primary residence.

  21. What is the maximum gross floor area allowed for an ADU?

    The ADU shall have a maximum gross floor area of 850 square feet.

  22. What is the maximum size for an ADU when attached to a garage or other accessory structure?

    An ADU attached to a garage or other accessory structure can also be up to 850 square feet.

  23. What are the size limitations for accessory structures like garages?

    Accessory structures such as detached garages can be up to 1200 square feet, while other accessory structures are generally limited to 800 square feet.

  24. How much covered space is allowed for porches or patios of an ADU?

    Covered porches or patios are limited to a total of 120 square feet and may not be enclosed.

  25. What is the minimum setback requirement from an interior side property line for an ADU?

    ADUs must be set back at least five feet from any interior side property line.

  26. What is the minimum setback requirement from a flanking street side yard for an ADU?

    ADUs must be set back at least ten feet from a flanking street side yard.

  27. What is the required setback if the rear yard does not abut an alley?

    If the rear yard does not abut an alley, the ADU must be set back ten feet from the rear property line.

  28. What is the maximum height allowed for a detached ADU?

    The maximum height for a detached ADU is 24 feet, measured at mid gable.

  29. Are there any height restrictions for accessory structures like garages?

    Maximum building height for detached accessory structures is generally 16 feet, measured at mid-gable, unless specified otherwise for specific uses.

  30. What are the fire wall requirements for garages near property lines?

    Garage walls on or within 5 feet of the property line must be built as fire walls.

  31. Can a detached garage be set on the property line?

    Yes, a detached garage can be set on the property line if the garage door is at a right angle to the alley entrance.

  32. What are the restrictions for windows in walls close to property lines?

    If within three feet of the property line, walls must have no windows or other openings.

  33. How far must a detached garage be set back if the door faces a side yard?

    If the garage door faces a side yard (flanking street), it must be set back 20 feet from that side yard.

  34. What is the setback requirement if garage doors face the alley?

    Garages with doors facing the alley must be set back 10 feet from the alley.

  35. How far must other structures be from other buildings on the same property?

    Structures such as a storage shed must be at least 6 feet from other structures.

  36. What is the maximum size for accessory structures in low-density residential zones?

    In low-density residential zones, each accessory structure shall not exceed 800 square feet in size, except for agricultural uses.

  37. Can accessory structures exceed 1200 square feet?

    Garages exceeding 1200 square feet per dwelling unit may be permitted as conditional uses in certain districts if they do not adversely affect the public interest and are compatible with the neighborhood.

  38. What are the specific conditions for allowing larger garages under conditional use permits?

    The Hearing Examiner establishes a maximum size for garages receiving conditional use approval, based on neighborhood compatibility and public interest.

  39. What is the setback requirement for a storage shed in the rear forty feet of a lot?

    A storage shed located within the rear forty feet of the lot must maintain a 10-foot setback from any flanking street side yard.

  40. Are there any numerical requirements for stairways to main ADU entrances?

    When an exterior stairway is needed to access the main ADU entrance, the use of open metal, prefabricated stairs is discouraged in favor of more integrated architectural features.

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