Digging into ADU and DADU Zoning Updates

It is becoming clearer than ever that this year is shaping up to be a landmark one for Seattle DADU and ADU development. That is thanks in great part to the recent passing of legislation in the state of Washington – House Bill 1337 and House Bill 1110. These housing bills both aim to expand housing options by removing some restrictions and permitting obstacles on the construction and use of accessory dwelling units in major metro areas across the state of Washington, including Seattle.

HB 1337 and HB 1110 are expected to go into full effect later this summer. This tandem of housing bills will make it easier than ever to plan and build both ADU and DADU additions in Washington and especially in Seattle, where housing options and solutions are highly sought after.

What is an ADU/DADU?

Before we get into the details of those bills, let’s get caught up on ADUs and DADUs.

A brief refresher from our blog post last month – an Accessory Dwelling Unit (ADU) and Detached Accessory Dwelling Unit (DADU) are legally permitted additional living units on your property. The main difference being that an ADU is “attached” to the main home – think a basement, or a new add onto the existing frame of a home.

A DADU is not connected to your main home, which can allow for more privacy and independent space. A DADU has a separate entrance, no shared walls with the main home, and potentially even a distinct address. A DADU is considered a permanent structure and can also be referred to as a backyard cottage, carriage house, guesthouse, or mother-in-law unit. These small, secondary homes—whether attached to your primary residence or built as standalone units—are a key part of the solution to the state’s housing shortage.

When it comes to the planning and building of an ADU or DADU in Seattle, homeowners have historically faced a lot of hang-ups that can stall the planning and building process. Homeowners and builders were met with ample zoning restrictions, red tape, and permitting processes that essentially made the process of building any ADU and DADU fairly difficult in the state of Washington. However, the recent passing of both HB 1337 and HB 1110 remove many of these obstacles for Seattle homeowners looking to an ADU and/or DADU onto their property.

Overview of Housing Bills 1337 & 1110

First, Washington House Bill 1337 (HB 1337) aims to expand housing options by easing barriers to the construction and use of accessory dwelling units (ADUs), requiring cities and counties to allow at least two ADUs per lot in urban growth areas and making other changes to ADU regulations. The target date for this to go into affect for the Seattle jurisdiction was February 2025 but is now expected to go into effect in the summer of 2025.

Additionally, HB 1110 also referred to as the “Missing Middle” housing bill, aims to increase housing density and affordability in the Seattle area as well as other major metro areas across the state of Washington.

Below, we will get into the details of both of these housing bills, the changes they expect to make in the Seattle area, and when those changes are expected to go into effect.

HB 1110: The Missing Middle Housing Bill

HB 1110 is expected to go into full effect near the end of July 2025. Signed by Governor Inslee in May 2024, Washington state’s “Missing Middle” Housing Bill mandates cities to allow more housing types, including middle housing, in areas that were previously zoned for single-family homes.

Seattle will implement HB 1110 through its One Seattle Comprehensive Plan. “Making progress on Seattle’s urgent housing affordability needs requires not only increasing overall production, but increasing the diversity of housing available,” Seattle Mayor Bruce Harrell said in a public statement. “ADUs are a key element of our One Seattle Housing Agenda as an efficient, sustainable option to address the housing shortage while also creating new opportunities for first-time homebuyers, multigenerational living, age-friendly housing, and financial stability for homeowners while also preventing displacement.”

The HB 1110 measure has a few key provisions, which are laid out in detail below.

• Increased Density: The bill requires cities to permit a minimum number of units on lots zoned predominantly for residential use in an aim to increase housing density.  It calls for cities to allow at least 4 units on all residential lots, and to allow at least 6 units on all residential lots if the lot is located within a quarter mile of a major transit stop (like King County Metro’s RapidRide lines) or if at least two of the units are affordable.

• Middle Housing Types: Cities must allow for “middle housing” types, which include duplexes, triplexes, four-plexes, townhouses, stacked flats, courtyard apartments, and cottage housing. 

• Urban Growth Areas: The bill applies to cities within contiguous urban growth areas, particularly those in and surrounding Seattle.

• Transit-Oriented Development: The bill encourages development near major transit stops, with provisions for higher densities within a half-mile of such stops. 

• Affordable Housing: The bill also encourages the inclusion of affordable housing units within middle housing developments. 

• Implementation Timeline: Cities are required to update their regulations to comply with the bill within six months after their city comprehensive plan updates. 

• Off-Street Parking: Off-street parking is not required for middle housing within a half-mile walking distance of a major transit stop. 

HB 1337 – Requirement Changes for DADUs and ADUs

HB 1337 will also help to ease prior restrictions on ADU and DADU construction. This bill requires cities and counties in the state of Washington to update their zoning and development regulations to comply with new statewide standards by June 30, 2025. Beginning July 1, 2025, homeowners will benefit from streamlined permitting, fewer restrictions, and more flexibility when building an ADU and/or DADU.

• Two ADUs Allowed: One attached and one detached.
• Parking Exemptions: No parking required near high-frequency transit routes.
• Design Flexibility: ADUs must meet basic design standards but no longer need to “match” the primary home.

Let’s get into the finer details of HB 1337. Below, is a more comprehensive list of the key changes that are going into effect under HB 1337 in July 2025.

• No More Owner-Occupancy Requirements: Starting July 1, 2025, HB 1337 eliminates the requirement that homeowners must live on-site to build or rent out an ADU or DADU. Ultimately, this means that as a homeowner, you can rent out both your primary home and your ADU or DADU independently and as separate units. This removes the requirement for annual affidavits proving that you live on your property with the exception of properties in environmentally sensitive areas, which still may have some restrictions.
• Two ADUs Allowed Per Lot: HB 1337 will allow homeowners to build two ADUs per lot, whether attached or detached. This means you can have one attached ADU (like a basement apartment) and one DADU (like a backyard cottage, or mother in law unit) OR you can have two DADUs, depending on your lot size and local zoning.
• No Size Limits Below 1,000 Sq. Ft: The permitting process for ADUs and DADUs has been simplified, with fewer restrictions on unit size and faster approval times. HB 1337 prohibits cities from setting maximum ADU sizes below 1,000 square feet. Homeowners now have more flexibility when designing their units, as size limitations have been relaxed to allow larger, more functional living spaces.
• Reduced Parking Requirements: Eliminating a major hurdle for homeowners in densely populated areas – HB 1337 no longer requires off-street parking if your property is within half a mile of a major transit stop. No longer needing to provide parking for your ADU or DADU can save homeowners from anywhere from $5,000–$15,000 in construction costs all while aligning with Seattle’s goal to reduce car dependency.

What does this all mean?

Ultimately, these two housing bills will allow for greater flexibility when planning to build an ADU or DADU in the Seattle area. Keep in mind that permit applications can spike when policy changes go into effect (see graph above), so this is a great window to begin thinking about building a Seattle DADU. We recommend working with a design-build firm to help you navigate these changes in zoning and permitting in order to build the Seattle DADU or ADU that works best for you.