Washington ADU Builders
Unlock Your Property’s Potential: Build Smart, Live Better
Why Build an ADU in Washington?
An Accessory Dwelling Unit is a secondary unit built on the same property as the primary home. Also known as a backyard cottage, in-law suite, guesthouse, and granny flat, the ADU can be attached or detached from the house.
A Landmark Opportunity for Homeowners
Today, Washington homeowners have a unique window of opportunity thanks to favorable housing legislation, a growing demand for rental units, and the chance to increase their property’s value.
With HiLine Homes, you can create a custom ADU that adds flexibility, equity, and income without the stress of navigating the process alone.
Financial Benefits of an ADU
Did you know that in Seattle, a 1-bedroom ADU can generate $1,400 to $3,000 a month in rental income? ADUs, particularly detached ADUs, are especially profitable in urban and high-cost markets, increasing the overall property value by 30-35%.
Housing Flexibility with an ADU Property
Accessory dwelling units serve a range of evolving household needs, from independent living for adult children and a ground-level home for elderly parents to home offices and guest accommodations. By adding an ADU, you can adapt your current property to your changing needs instead of buying a larger home or relocating.
Navigating Washington's New ADU Regulations: A Simple Guide
Washington ADU laws are complex. In fact, they can be intimidating for the average homeowner. With Hiline Homes, we help you navigate these laws and make ADU development easy to understand and implement. One of these laws is HB 1377 or RCW 36.70A.681. This law took effect on July 1, 2025, expanding housing opportunities with reduced barriers for homeowners to build ADUs.
Feature | Old Rule (Typical Pre-2025) | New Rule (Effective July 1, 2025) | What This Means for You |
|---|---|---|---|
Owner Occupancy | Often required on the property | Not required | Greater flexibility; rent out both your main home and the ADU for maximum income. |
Number of ADUs | Typically one per lot | Up to two per lot allowed | Double your rental potential or create a family compound with separate living spaces. |
Maximum Size | Often limited to 800 sq. ft. or less | Cannot be limited to less than 1,000 sq. ft. | Build a more spacious, comfortable, and valuable 2-bedroom ADU that attracts higher rent. |
Parking | Often 1 off-street space required | No parking required near major transit; 1 space max on smaller lots | Save thousands in construction costs for driveways and parking pads, and simplify your site plan. |
Separate Sale | Not allowed | Allowed (as a condominium) | A new way to build and sell smaller, more affordable homes, unlocking a different kind of equity. |
Impact Fees | Full or variable rates | Capped at 50% of the primary home’s fees | Significant and direct savings on municipal permitting and development costs. |
With this new law, you’re in a position to capitalize on the positive changes made that amplify your property’s buildability and income potential. For assistance with your local county rules, contact us to learn how the law applies to your specific property and project.
Our Washington ADU Plans & Building Services
Unlike tiny homes on wheels or modular units, HiLine’s custom-built ADUs don’t depreciate. Because it has the same equality materials, housing code requirements, and construction standards as the main dwelling, it offers long-term value and equity growth. Below are examples of our stick-built ADU home plans that homeowners can select to maximize their property’s value and income potential.
Home Plan 800
Home Plan 800 (1-bed, 1-bath, 800 sq. ft.) is ideal for maximizing space under HB 1337’s 1,000 sq. ft. size allowance.
Home Plan 500B
Plan 500B is compact yet highly functional, perfect for guests, offices, or supplemental income.
Our ADU plans can be built attached or detached from a HiLine Home build. For example, the 500B can be attached to larger home designs, such as the two-story Plan 1768, to support multigenerational living or dual-rental setups.
On Your Lot, On Time, Built Right!
Need an ADU plan to fit into your local zoning and site conditions? Let’s customize your desired ADU plan to make it possible.
Download Our Accessory Dwelling Unit Guide!
Shell and Super Shell Programs for More Savings and Sweat Equity
At HiLine Homes, we also offer Shell and Super Shell programs for customers who prefer to manage the interior build-outs themselves. Ideal with DIY-ers and people with experience in interior construction, these programs enable future homeowners to save on home-building costs while benefiting from a strong foundation and exterior build quality from HiLine.
ADU Hotspots: Where to Build in Washington
At HiLine Homes, we understand the local zoning, market conditions, and construction trends across Washington. Here are the Washington hotspots for building ADU rentals.
Puget Sound (Seattle, Bellevue, Tacoma)
Seattle’s high home prices and rental rates make its ADUs a profitable income stream, with a rental income potential of $1,500 to $3,500 monthly. With Puget Sound’s competitive rental markets and high property appreciation, ADUs are guaranteed as long-term real estate investments for years to come.
University Towns (Bellingham, Pullman, Ellensburg)
ADUs allow adult children to live independently while living close to home, especially while they are still college students. For homeowners in university towns, students, faculty, and visiting scholars provide year-round demand, ensuring consistent occupancy and fewer tenant turnover issues with their ADU rentals.
Tourist Destinations (Leavenworth, Port Angeles, Long Beach, Forks)
Beach cities are vacation-friendly areas with strong short-term rental (STR) demand. Many ADU owners in these areas report high annual revenues, especially during peak travel seasons.
Suburban Growth Areas (Clark County/Vancouver, Spokane)
The influx of new residents from California and Oregon increases the demand for rental housing in Washington. ADUs offer affordable housing solutions in areas with limited inventory. With a strong long-term growth trajectory in the housing market, now is a good time to build.
Understanding the Cost to Build an ADU in Washington
ADU construction costs can vary widely based on location, size, finishes, and site conditions. Additional factors, such as permits, site preparation, and landscaping, can drive up the costs of the investment for a basic structure. By partnering with a custom ADU builder, you can acquire accurate prices for your ADU build with realistic numbers and fewer surprise costs.
Financing Your Washington ADU Project
HiLine Homes maintains a network of Approved Lenders who are familiar with our ADU plans and building packages. Select lenders offer up to 100% financing, based on the appraised value of the completed home.
By working with our lenders, you streamline your application, reduce the back-and-forth, and ensure your ADU project stays on track. Contact us to discover which ADU financing program is right for you with our pre-qualification form.
Start Your ADU Project with Washington's On-Your-Lot Experts
As Washington’s trusted ADU builder, our decades of experience allow us to help homeowners unlock their property’s full potential. Through our on-your-lot building model, our team will guide you on flexible, cost-saving, and site-specific solutions for your ADU construction project, helping you achieve rental income, maximized living space, and more.
Contact us today to schedule a consultation and discover what’s possible on your Washington property.