Washington Law on Creating a Mother-in-Law Apartment to Rent

Before you start building or knocking down walls, find out the relevant rules for Washington State homeowners.

Developing a mother-in-law apartment for rent may be an option for Washington homeowners seeking additional income or another living option for family members. But before building or remodeling, check whether this type of dwelling unit is allowed by local laws at the city or county level. If you are not careful, your construction could violate the law, and you might face permit requirements and fines.

To avoid these potential pitfalls, first confirm whether your community’s zoning ordinances allow development of mother-in-law apartments or similar units on your property. In Washington state, land within an incorporated municipality or unincorporated land within a county is classified into zoning districts. The types of uses allowed within a zoning district and the restrictions on allowed uses are listed within the zoning ordinances. Local zoning ordinances are mostly available online through your city or county website.

Find your property on the municipal or county land use map available on your city or county website, look up your zoning classification on your tax assessor’s parcel report, or ask your local planning or development services department to confirm your zoning district. Check whether a mother-in-law apartment or similar unit is allowed within your zoning district.

If a mother-in-law apartment or similar unit is allowed on your property, look again to your community’s zoning ordinances to confirm exactly how large the unit can be (in terms of total square footage, building footprint, or habitable area), what the unit can look like, and where the unit can be placed on your property. These ordinances may also affect where exactly you can place the unit on the property, to comply with lot line setbacks or buffers and building setbacks from critical areas or shorelines.

You may also have to work within limits on the total amount of your property that can be developed, the total amount of impervious surface (such as concrete or asphalt) that can cover the property, maximum building height, and types of permissible architectural design. If you are in a historic district, for instance, you may not be able to build a modern-style unit. Also check on any requirements for operating a mother-in-law or similar unit on your property, like proof of water and sewer service or approved septic design, emergency access and fire protection, vehicle access and parking, and landscaping.

Even if the mother-in-law unit is allowed on your property and you’re sure that you’ve met all zoning ordinance requirements, you may be required to obtain one or more permits before someone moves in. Again, check your community’s zoning ordinances to confirm.

And that's not all: locating the unit on the property may require a special zoning permit, such as a conditional use permit. Washington state is also quite protective of its shorelines, so any waterfront development may require a shoreline substantial development permit. Any rehabilitation, renovation, or construction activity involved in creating the unit may require building permits, grading permits, or other site development activity permits. (If you hire a contractor, he or she should be aware of and able to help obtain these.) Depending on the vehicle access to the unit, a right-of-way permit may also be required. Some types of permits require applicants to attend a public hearing on the permit request.

Visit your local planning department for specific information on permits, fees, and other review required. Failing to make these inquiries puts you at risk of fines, enforcement action, and possible loss of your investment. For legal advice, contact an attorney who has local land use development experience.

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