Modular accessory dwelling unit — Federal Way, WA
Last updated: September 24, 2026
What affects the price
Building an ADU involves several moving parts that shape your total investment. The final figure depends heavily on whether you choose a prefab unit or a custom build from the ground up. Site conditions are another major factor; if your property requires extensive grading, utility trenching, or foundation work, costs will climb. You also have to consider your choice of finishes, like high-end appliances or custom cabinetry. Navigating local permit fees and impact assessments also adds to the total.
About this service
A modular unit is built in a factory and then transported to your property to be finished on a foundation. You should consider this if you want a faster construction timeline with less disruption to your daily life. It is an efficient way to add high-quality space to your lot.
Frequently asked in Federal Way
Where can I find accessory dwelling unit builders near me in Federal Way?
What's the accessory dwelling unit construction cost in Federal Way?
What ADU services are available in Federal Way?
Who is an ADU builder in Federal Way?
What is an accessory dwelling unit (ADU) in Federal Way?
What does accessory dwelling unit mean for Federal Way homeowners?
What is accessory dwelling unit Massachusetts?
Also serving nearby: Tacoma · Kent · Renton · Seattle · Bellevue
More on this: HUD ADU resources — ADU policy and financing research.
Service area details
Serving all of Federal Way — population 97,701; about 4,375 residents per square mile across 22.3 sq mi. ZIP codes covered include 98003, 98023, 98063, 98093.
Neighborhoods served in Federal Way: Mirror Lake, West Campus, The Lakes, Woodmont, Arrowhead — and every surrounding area.
Areas we serve
Browse by state, then city.