In Summary:
- Washington's HB 1337 requires cities and counties to allow at least two ADUs (attached, detached, or one of each) on single-family lots in urban growth areas, and bans owner-occupancy requirements.
- Whether your Gig Harbor-area home follows City of Gig Harbor or unincorporated Pierce County rules makes a big difference, so confirm your parcel's jurisdiction with the right planning department before you plan anything.
- ADUs can house family, bring in rental income, or support house hacking. Local building, septic, and short-term-rental rules still apply, so treat rental income as an estimate, not a guarantee.
In 2024, the Federal Reserve Bank of San Francisco estimated that there were at least 1.6 million accessory dwelling units (ADUs) in the US. An ADU is basically a small yet independent housing unit on a single-family property, and as you can imagine, it can be very useful whether you want to house loved ones or make extra profit.
Are you considering adding an accessory dwelling unit in Washington State? Then you need to know about the new state ADU law, which may change what you can or can't do with your property.
What Changed? A Quick Overview of HB 1337
Washington's ADU law, House Bill 1337, took effect on July 23, 2023, and changed the baseline rules for ADUs in many urban areas.
In general, the law says that in applicable cities and counties, you're allowed to have at least two ADUs per single-family lot within urban growth areas. Depending on your specific configuration, this can be:
2 attached units
2 detached units
1 attached unit, 1 detached unit
One of the biggest benefits of this ADU Washington State law is that it prevents local governments from requiring property owners to live in either the main home or the ADU, which means that ADUs are much more flexible now for Gig Harbor homeowners than before. Do note that cities can still limit ADUs from being used as short-term rentals.
Does This Apply to My Property in Gig Harbor?
Does your house have to follow accessory dwelling unit rules? Well, it depends. The first and foremost question is this: which jurisdiction governs your property?
For example, if your home is inside the City of Gig Harbor, then it's regulated through the city's municipal code and planning processes. The City of Gig Harbor updated its code to comply with HB 1337 in 3 May 2025, so these rules are still fairly new locally
On the other hand, if it's in unincorporated Pierce County, then it falls under Pierce County's development regulations. In that case, the rules there describe separate urban and rural ADU standards, and there are different maximum sizes depending on whether your house is inside or outside the urban growth area.
As you can imagine, that distinction really matters for Gig Harbor-area properties since homes just a few blocks away from one another can be in different jurisdictions. This means that before you plan an ADU, you should contact the planning department for either the City of Gig Harbor or Pierce County and confirm the rules.
The following can all affect what's possible:
Zoning
Lot characteristics
Critical areas
Utilities
Why Homeowners Are Adding ADUs
There are so many benefits you can get by adding an ADU, so let's go over them.
For one, do you want to keep your family members close, but don't have enough space in the house? Or do your loved ones prefer to have more privacy?
Either way, an ADU (either attached or detached) can help. For instance, you can give your aging mother a private living space while she stays close, for your peace of mind, or you can give your adult child more independence without paying for a separate apartment elsewhere. You can even use the ADU as a guest space for now, then adapt it later as your needs change.
Besides having a mother-in-law suite in Gig Harbor, you can also get ADU rental income. Make sure to check the applicable local rules and market conditions first, but you can make some extra money by renting out your ADU space.
There's another possibility: house hacking real estate. This is where you live in one unit while renting out the other. Since HB 1337 removed the owner-occupancy requirements, you'll have more flexibility too.
What to Know Before You Build
Yes, HB 1337 removes or limits certain local barriers, but you'll still have to comply with building, zoning, environmental, and utility requirements.
I want to point out that Washington specifically allows local governments to apply public health, safety, building code, and environmental requirements, and this includes protections related to onsite wastewater systems. This is really relevant around Gig Harbor, since homeowners often have properties that have septic systems or wells.
In my experience, homeowners who work with local contractors have a much easier time. They'll be familiar with the rules, and they can identify any practical construction issues before you even start. You should still confirm the regulatory requirements directly with the appropriate planning and permitting department though.
What HB 1337 Doesn't Change for Gig Harbor Homeowners
It's important to know that this law doesn't just turn ADUs into completely unregulated additions. Local governments can still apply certain development standards, as well as some restrictions on ADUs being used as short-term rentals.
This means that whether you're building an ADU for your elderly parent or planning to use one as an Airbnb, you should still confirm with the planning department. Finding out which current rules apply to your parcel itself can make a huge world of difference before you proceed with expensive building.
How ADU Rules Can Affect Your Investment Strategy
If you're considering investing in Gig Harbor properties, then the ADU rules can become part of a property's overall evaluation. For example, if you find a home that looks suitable for an ADU, then not only can you generate rental income from it, but you can also use it for multigenerational living or house hacking.
When you're looking at properties, consider them as a whole. Factor in:
Jurisdiction
Zoning
Lot configuration
Access
Utilities
Physical constraints
Don't automatically assume you'll get a return on potential rental income either. Instead, treat it as an estimate to avoid disappointment and financial surprises.
How an ADU Can Affect Your Home's Value
An accessory dwelling unit in Washington State can have a positive effect on your home's value. Not only will it raise its resale appeal, but it can also give you good rental income potential.
Are you curious about whether your property is a good ADU candidate? Then contact me now. I'll put my real estate experience and knowledge to good use for you!
