What Is an Accessory Dwelling Unit?
An Accessory Dwelling Unit (ADU) is an independent living space located on the same property as an existing single-family home. These units — sometimes called in-law suites, granny flats, or backyard cottages — can take several forms:
- Detached ADU: A standalone structure separate from the main house
- Attached ADU: Built as an extension or conversion of the primary residence
- Garage conversion: An existing garage renovated into a livable unit
ADUs offer homeowners flexibility — whether for housing family members, generating rental income, or simply adding usable space to a property. For communities, they represent a practical way to address housing shortages without expanding into undeveloped land.
JayMarc Homes, a custom home builder operating in Washington State, has been paying close attention to the growing interest in ADUs. According to Olivia Horine, Interior Designer at JayMarc Homes, ADU permits in the Seattle area increased by 250% between 2019 and 2022 — a trend that reflects both rising housing demand and a shifting regulatory environment.
Why ADUs Are Growing in Popularity in Washington State
Washington State is home to more than 8 million residents, and the population continues to grow. A study by the Building Industry Association of Washington (BIAW) found that the state needs at least 251,894 new housing units to meet current demand. ADUs are increasingly seen as part of the solution, allowing better use of existing properties without requiring large-scale development.
By March 2023, builders in Washington were constructing more ADUs than new single-family homes. This shift is partly driven by HB 1337, signed into law in June 2023, which introduced significant policy changes making it easier to build ADUs across the state. Local governments are now required to relax zoning restrictions, reduce parking requirements, and streamline ADU approval processes.
Key points from the new Washington State ADU rules include:
- Every lot in a GMA urban growth area must allow at least two ADUs in addition to the primary residence
- Local governments cannot set ADU size maximums below 1,000 square feet
- ADUs must follow the same dimensional standards as the principal home — no stricter setback or yard coverage rules
- Street improvements such as road widening can no longer be required as a condition for ADU permits
- Owner-occupancy requirements have been removed, allowing homeowners to rent both the main home and the ADU
- ADUs within half a mile of major transit stops are exempt from on-site parking requirements
- Impact fees for ADUs are capped at 50% of those assessed for the primary residence
- New homeowner associations cannot adopt rules prohibiting ADU construction
It's worth noting that while the law allows up to two ADUs per lot, homeowners are not required to build one. The legislation simply removes barriers for those who choose to do so.
Pros and Cons of Adding an ADU to Your Property
Before moving forward with an ADU builder like JayMarc Homes, it's useful to weigh the practical advantages and challenges involved.
Potential benefits include:
- Flexible living arrangements for family members or guests
- Additional rental income, with no owner-occupancy requirement under the new rules
- Increased overall property value
- More cost-effective when integrated during new home construction, allowing shared utility and foundation work
- Sustainable land use within existing neighborhoods
Considerations to keep in mind:
- Upfront construction costs — materials, labor, permits, and utility connections — can be significant
- Lot size and setback requirements may limit design options on existing properties
- Utility infrastructure, including water, sewer, and electricity, can add complexity and cost
- An ADU — whether attached or detached — may affect outdoor space or privacy
- Renting an ADU comes with ongoing responsibilities such as tenant management and maintenance
JayMarc Homes works with homeowners looking to incorporate an ADU as part of a broader custom home build in Washington State. For those considering a detached ADU, garage conversion, or an in-law suite built into a new home, exploring options early in the planning process can help manage costs and design constraints more effectively.
