Washington's New ADU Laws: Build Up to 6 Units Per Lot

Washington's New ADU Laws: Build Up to 6 Units Per Lot

Washington State has enacted some of the most aggressive ADU and middle housing legislation in the country. Two landmark bills — HB 1337 and HB 1110 — are fundamentally changing what homeowners and builders can do with residential property across the state.

HB 1337: ADU Reform

Effective since 2023 and now fully implemented statewide, HB 1337 requires all cities planning under the Growth Management Act to allow at least two ADUs per single-family lot — one attached and one detached, or two detached units.

Key provisions:

This means a homeowner with a single-family lot can now have their primary residence plus two additional rental units — without living on the property themselves.

HB 1110: Middle Housing

HB 1110 goes further, requiring cities to allow middle housing — duplexes, triplexes, fourplexes, and cottage clusters — on lots traditionally zoned for single-family homes.

For cities with populations over 75,000:

• 4 units per lot in all residential areas
• 6 units per lot within 1/4 mile of a major transit stop

For cities with populations between 25,000 and 75,000:

• 2 units per lot in all residential areas
• 4 units per lot near transit

What This Means for Builders and Investors

The combination of HB 1337 and HB 1110 creates significant opportunities:

• Builders can develop multiple units on single lots without rezoning battles
• Investors can acquire single-family properties and add density for rental income
• Homeowners can build DADUs (Detached Accessory Dwelling Units) to generate income or house family members
• No owner-occupancy requirement means investment properties can include ADUs

For contractors specializing in DADU construction, the addressable market has expanded dramatically. Every single-family lot in Washington's major cities is now a potential multi-unit development site.

Construction Considerations

Typical DADU construction costs in Washington range from $150–$350 per square foot depending on size, finishes, and site conditions. A 600 sq ft DADU might cost $90,000–$210,000 to build, while generating $1,500–$2,500 per month in rental income in the Puget Sound region.

For homeowners who want to build but can't find traditional financing, iCAP Underwriter can restructure the financial plan to identify viable funding pathways — whether through conventional lending, home equity, or alternative financing structures.

The Bottom Line

Washington's ADU and middle housing laws are among the most permissive in the nation. For builders, investors, and homeowners who understand the opportunity, the window to add housing supply — and build wealth — is wide open.

Sources: Washington State Legislature HB 1337 (2023); HB 1110 (2023); Municipal Research and Services Center of Washington; Harjo Construction ADU/DADU Zoning Guide (March 2025).