Looking for an ADU builder in Federal Way, WA? Describe your project once and up to three licensed ADU builders who work in Federal Way and King County get back to you with a quote. It's free and there is no obligation.
Federal Way is home to about 100,252 people (2024 Census Bureau estimate), a stable community (−0.8% since 2020). With 4,493 people per sq mi, it is a densely built-up area. Homeowners here add accessory dwelling units for rental income, family members or a home office.
Federal Way at a glance
- Population (Census Bureau estimate, 2024)
- 100,252 (−0.8% since 2020)
- Rank in Washington
- 10th of 281 cities and towns
- Land area
- 22 sq mi
- Population density
- 4,493 people per sq mi (densely built-up)
- Type
- City (Federal Way city)
- Main county
- King County
What to check before hiring in Federal Way
- Rules: ADU rules are set by the state and the city; check Federal Way's planning department for size, setback and parking limits.
- License and permit: in Washington, ADUs are built by licensed contractors under a building permit.
- Utilities: water, sewer and electrical capacity can drive the cost as much as the unit itself.
- Compare: ask for references and photos of ADUs each builder has completed.
Useful official resources
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HUD User
Federal housing research, including ADUs.
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ICC: International Code Council
Publisher of the building codes most states and cities adopt.
Services available in Federal Way
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Detached ADU
Design and construction of a detached backyard unit, site-built or prefab, with the permit and utilities.
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Garage or interior conversion
Conversion of a garage, basement or part of the house into a legal accessory dwelling unit.
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ADU plans and permit
Feasibility, plans and permit application for an ADU before choosing a builder.
Frequently asked questions in Federal Way
How much does an ADU cost in Federal Way, WA?
It depends on the type of unit, its size, the finishes and the utilities. Describe your project: builders who serve Federal Way reply with a quote.
How long does an ADU take?
Often several months for permits and a few more for construction; conversions and prefabs can be faster.
Do I need an architect?
Not always: designers, builders with in-house design or prefab companies can prepare the permit set, depending on local rules.
Which licensed ADU builders will contact me?
Up to three independent licensed ADU builders who serve Federal Way and the surrounding area, chosen for the service you need. Your details are shared only with your consent.
Source: U.S. Census Bureau, Population Estimates Program (Vintage 2024) and Gazetteer Files (public domain), imported on 10/31/2026. Figures describe the incorporated place or county as published; they are shown for information only.