New middle housing legislation in Washington state is making it easier for property owners to build accessory dwelling units (ADUs). Beginning in June 2025, Seattle and other cities across the state will implement updated zoning and land use regulations in single-family neighborhoods to support more housing development. This legislation aims to expand housing options, create new development opportunities, and help address future housing shortages in densely populated areas.
At Haas Development and Construction, our certified ADU/DADU specialists are ready to answer your questions about the new regulations and what they could mean for your next ADU or DADU project.
What is Middle Housing?
Middle housing refers to housing types that fall between single-family homes and large multifamily buildings. This includes multi-unit structures like townhomes, duplexes, triplexes, and fourplexes, as well as smaller units such as cluster homes, cottage housing, and both attached and detached accessory dwelling units (ADUs and DADUs). The goal of middle housing development is to increase affordable housing options, promote smart urban density, and offer more diverse living arrangements. New legislation in Seattle and across Washington state supports these goals by giving property owners greater flexibility in how they use their land.
Updates to Middle Housing Legislation
The City of Seattle, along with other cities in the Puget Sound region, has adopted House Bill 1110 and House Bill 1337—two pieces of legislation that ease restrictions on building ADUs, DADUs, and other forms of middle housing. Previously, strict regulations in Seattle often made it difficult for property owners to move forward with ADU and DADU construction. These new laws aim to remove many of those barriers. Under House Bill 1110, depending on the size of the lot, property owners in Seattle can now build up to two DADUs and have up to four units per lot, regardless of existing zoning. House Bill 1337 removes several long-standing requirements that limit middle housing development. Key changes include eliminating the owner-occupancy rule, removing off-street parking requirements, reducing DADU-related impact fees, and expanding design and development options for DADUs.
Building a Seattle DADU in 2025
The new legislation not only simplifies the process for building additional housing on residential lots—it also expands what property owners are allowed to build. Starting in June 2025, Seattle will increase the maximum allowable size for detached accessory dwelling units (DADUs) from 1,000 to 1,500 square feet. This change makes it possible to build larger, three-bedroom DADUs—opening the door to more flexible living arrangements, greater investment potential, and increased housing access in single-family neighborhoods.
At Haas Development and Construction, we’re excited to debut our newest three-bedroom DADU floorplan—designed to help homeowners take full advantage of these expanded options for larger DADU living.
Getting Started on Your DADU Project
If you’re considering adding middle housing to your residential property and want to explore your options, the team at Haas Development and Construction is here to help. With years of experience in ADU/DADU construction, residential remodels, and home development, we’ll guide you through the possibilities and benefits of middle housing tailored to your property. Building an ADU or DADU can provide rental income, create affordable options for first-time homebuyers, and support aging in place—all while increasing your property’s value and versatility. Contact our ADU/DADU specialists today to start planning your project!
