Accessory Dwelling Unit (ADU) Contractor Serving King County

If you own a home in King County, you’re sitting in one of the most expensive real estate markets in the United States. Whether you’re looking to generate rental income, provide housing for family members, or increase your property value, an Accessory Dwelling Unit (ADU) is a powerful option—and Washington’s HB 1337 (effective July 1, 2025) makes it more accessible than ever.

Prolific Design-Build and Restoration specializes in full-service ADU construction for homeowners throughout King County. We handle every phase from permits and site preparation through framing, drywall, painting, concrete, and final finishing.

Why Build an ADU in King County?

Rental Income: A studio or one-bedroom ADU in King County can generate $1,500–$2,500 monthly in rental income depending on location and finishes.
Home Value: In Bellevue, Sammamish, and other high-value areas, an ADU can add $150,000–$250,000 in property value.
Family Housing: Build independent housing for aging parents, adult children, or extended family without relocating.

What HB 1337 Means for You

HB 1337 now permits two ADUs per residential lot, streamlines permitting, and reduces parking requirements. Projects that used to take 18 months to permit now move through approval in 6–8 months in many King County jurisdictions.

ADU Cost Estimates for King County

Detached ADU (800–1,200 sq ft): $180,000–$300,000 including foundation, framing, roof, siding, all systems, and finishes.
Attached or Garage Conversion (400–700 sq ft): $120,000–$200,000.

Our Full-Service ADU Build Process

Site assessment and planning → Design and permitting support → Foundation and concrete → Framing and structural → Drywall and interior finishing → Exterior siding and painting → Trade coordination → Final inspection and turnover.