ADU in Washington State

ADU Laws in Washington State

Byaaronrobinson
ADU Laws in Washington State: What Bothell Homeowners Need to Know | Aaron Robinson
Market Updates

ADU Laws in Washington State: What Bothell Homeowners Need to Know to Maximize Property Value

ADU and DADU get thrown around like nouns these days. Before you order that lumber, here's what the rules actually say, what Bothell specifically allows, and what most people get wrong.

By Aaron Robinson  ·  Keller Williams Realty Bothell  ·  May 2025

ADU DADU laws Washington State Bothell homeowners property value

ADU and DADU are being thrown around like regular nouns these days. I hear it from investor clients, from other agents, from builders, from people who just watched a show on HGTV and decided their backyard has untapped potential. The flippers especially love this conversation. And honestly, they are not wrong to.

But here's the thing. There is a meaningful difference between knowing what an ADU is and knowing what you are actually allowed to build, how big it can be, what you can use it for, and whether your lot and your city even support it. Most people are somewhere in the middle of that gap. They know the concept. They do not know the specifics. And the specifics are where this either makes money or makes problems.

So let's close that gap. If you own a home in Bothell, or you are looking at buying one and ADU potential is part of your calculation, here is what the ADU laws in Washington State actually say, and what Bothell specifically allows.

2 ADUs now allowed per residential lot in Washington State urban growth areas, per WA Dept. of Commerce, HB 1337
1,000 sf Minimum ADU size local governments must allow, per RCW 36.70A.681
$0 Owner-occupancy requirement. State law removed it. You can rent both units, per MRSC.org
50% Maximum impact fee cap for ADUs vs. the primary unit, per RCW 36.70A.681

Figures per Washington State Department of Commerce and MRSC.org as of 2025. Bothell-specific rules referenced from bothell.municipal.codes. Always verify current code with the City of Bothell Planning Department before beginning any ADU project.

ADU, DADU, JADU: What the Acronyms Actually Mean

Most people recognize the concept, even if they have never used the acronym. The old term was mother-in-law suite. The unit usually has its own bed, bathroom, and sometimes a kitchen. It lives on the same lot as the primary home but functions as a separate dwelling space. That is the concept. The acronyms just describe where it sits.

The Four Types

Know what you are talking about before you talk to a builder.

  • ADU (Accessory Dwelling Unit): The umbrella term. Covers anything that qualifies as a secondary self-contained living space on a residential lot.
  • AADU (Attached ADU): Connected to the primary home, sharing at least one wall. Basement conversions, garage conversions on the side of the house, additions. More cost-effective because you leverage existing structure.
  • DADU (Detached ADU): A standalone structure in the backyard, fully separate from the primary home. Its own walls, its own roof, its own entrance. More expensive to build, more private to live in, and generally more desirable from a rental standpoint.
  • JADU (Junior ADU): A smaller unit of up to 500 square feet created within your existing home's footprint. Must include a cooking facility. Can share a bathroom with the main house. In Bothell, per GatherADU.com, you can build both a JADU and a full ADU on the same property, though JADUs require owner-occupancy in either the main house or the JADU.

What Washington State Law Now Requires

Washington State passed HB 1337, codified as RCW 36.70A.680-681, and it changed the landscape significantly. This is state law, which means it sets the floor. Individual cities can be more permissive, but they cannot be more restrictive than what the state requires.

State Law: HB 1337

The rules every Washington city must follow, per the Washington State Department of Commerce.

  • Two ADUs per lot required: All cities within urban growth areas must allow at least two ADUs per residential lot, in addition to the primary residence, for lots that meet minimum size requirements. Those two can be any combination of attached, detached, or existing structure conversions.
  • Minimum size of 1,000 square feet: Local governments cannot require ADUs to be smaller than 1,000 gross square feet. Cities can allow larger; they cannot cap smaller.
  • Owner-occupancy removed: The state has removed the owner-occupancy requirement. You do not have to live on the property to rent out both the primary home and the ADU. Note: some short-term rental restrictions may still apply at the city level.
  • Parking restrictions limited: No on-site parking can be required for ADUs located within half a mile of a major transit stop, per RCW 36.70A.681. Elsewhere, the cap is 1 space per unit on lots under 6,000 square feet.
  • Impact fees capped at 50%: ADU impact fees cannot exceed 50% of what is charged for the primary dwelling unit.
  • Height and setbacks at parity: Height limits, setback requirements, and design standards for ADUs cannot be more restrictive than what applies to the primary residence on the same lot.
The Preemption Rule

If a city's code conflicts with state law and the city has not updated its regulations by its Growth Management Act deadline, the state statute preempts the local code. In practice, this means state law is the fallback. But always verify with your city's planning department before breaking ground, because local code is what the permit desk will be reading.

What Bothell Specifically Allows

This is where I always tell people to slow down. State law tells you what cities must allow. Bothell's municipal code tells you what Bothell actually permits, and those are two different conversations. I drive past Parr Lumber on my way to Mill Creek more often than I care to admit, and every time I do, I think about how many people have ordered materials before they had the permit conversation. Do not be that person. Know what Bothell allows before you design anything.

The good news is that Bothell has been moving in a more permissive direction on ADUs for several years. The city updated its ADU regulations as part of its affordable housing strategy, and the rules have opened up meaningfully. Here is what the current Bothell code actually says.

Bothell Municipal Code — bothell.municipal.codes/BMC/12.14.135

Bothell ADU Rules as of 2025

Per HouseHack Seattle and Bothell Municipal Code Section 12.14.135, the current framework for ADUs in Bothell includes:

  • One AADU or one DADU allowed in most standard residential configurations (state law is pushing this toward two, so confirm current status with the City of Bothell Planning Department)
  • Size limit: The floor area of the ADU can be no more than two-thirds of the primary dwelling unit, or 40% of the total floor area of the primary dwelling unit and ADU combined, excluding garage area, or 800 square feet, whichever is less. Note that state law requires cities to allow at least 1,000 square feet, so this is an area where Bothell's local code and state requirements may intersect, and current code should be confirmed directly.
  • Detached ADU height: No greater than 25 feet, or 28 feet for units built over an existing accessory structure
  • Two-story DADUs: Specific privacy rules apply when building two-story detached units to protect adjacent properties
  • One additional parking space required
  • ADUs are exempt from provisions limiting accessory building size to 5% of lot area
  • ADUs are allowed on lots developed with middle housing, per the MRSC code reference for Bothell Municipal Code Sec. 12.14.135
  • Garage conversions over alleys allowed, following a code update that specifically enabled this configuration
Always Verify Directly

Bothell's ADU code has been updated multiple times and will continue to evolve as the city works toward GMA compliance. The rules above reflect published code as of 2025. Call the City of Bothell Planning Department at (425) 806-6400 before finalizing any project scope. The permit desk is the final word.

The Airbnb Question: What You Can and Can't Do

The Airbnb crowd gets excited about ADUs for obvious reasons. A detached backyard cottage on a quiet Bothell street, close to the Burke-Gilman Trail, is a legitimate rental product. That part is real.

Here is what you need to know before you build for short-term rental income.

Short-Term Rental Rules

State law doesn't prohibit it. Local government can.

Per Steadily.com's Washington ADU analysis (April 2026): state law does not prohibit short-term rentals from ADUs, but local governments have the authority to restrict them. Some Washington cities, including Bellingham and Poulsbo, have banned ADUs from operating as short-term rentals entirely.

Bothell has its own short-term rental regulations separate from the ADU code. If your plan involves Airbnb income, verify the current STR rules with the City of Bothell before your project scope is locked. Building a DADU optimized for short-term rental income and then discovering the city restricts it is a very expensive surprise.

If your plan is long-term rental income, you are on firmer ground. With the state's owner-occupancy requirement removed, you can rent both the primary residence and the ADU without living on the property. That is a meaningful shift from where the rules were even a few years ago.

What It Costs and How People Finance It

Let's talk numbers. Because the conversation about whether an ADU makes financial sense starts and ends here.

Cost Reality Check

The range is wide. The variables are real.

Per Goodwin Construction's 2025 DADU guide, construction costs for detached ADUs in Washington State typically range from $250,000 to $450,000. A more modest 600-square-foot DADU in Western Washington runs approximately $150,000 to $250,000 all-in, while a 1,000-square-foot unit in a higher-cost market can reach $300,000 or more, per Steadily.com.

The variables that move that number most significantly:

  • Site conditions. Sloped lots cost more, full stop.
  • Utility connections, often the largest single variable
  • Finish level and design complexity
  • Whether you are building new or converting an existing structure
  • Permit timeline, which affects carrying costs

Realistic timeline for a new DADU in most Washington markets: design and site planning takes one to two months, permit review runs six to sixteen weeks depending on jurisdiction and whether you use pre-approved plans, construction adds three to six months, and final inspection comes last. Budget ten to fourteen months from project start to first rent check, per Steadily.com.

Financing options most homeowners use: Home equity loans or HELOCs against the existing property, construction loans that convert to permanent financing, and cash-out refinances. Washington State also offers a 3-year property tax exemption for qualifying remodeling projects that include ADU construction, per the Washington State Department of Commerce. That is worth a conversation with your accountant before you build.

Wondering If Your Bothell Property Can Support an ADU?

I can walk you through the lot and help you figure out what makes sense, before you spend a dollar on design. That conversation is free. The information is worth a lot more than that.

Talk to Aaron Read: ADU vs. Multifamily Investment

What an ADU Does to Your Property Value

Here's what I would say about that: it depends more on execution than existence.

A permitted, well-built ADU that functions as a legitimate rental unit adds to your property value in two ways. It expands the buyer pool when you sell, because you are now selling to investors and house hackers and multigenerational buyers in addition to traditional owner-occupants. And it provides income that a sophisticated buyer will factor into their offer calculation.

An unpermitted ADU does the opposite. It creates liability, it complicates the sale, and it can require remediation before closing. I have seen deals fall apart over unpermitted units. The cost of doing it right up front is almost always less than the cost of cleaning it up after the fact.

The multigenerational angle also has real value in the Bothell market specifically. Eldercare costs are real, and a separate space for aging parents or a child who is not quite financially independent is a meaningful quality of life asset. Those buyers, people who want the space without the separation, pay for that value. It is not as easy to quantify as rental income, but it is real.

The Right Order of Operations Before You Build

Do This Before You Order Lumber

The sequence matters as much as the rules.

Step 1: Verify your lot and zoning. Not every Bothell lot is the same. Lot size, setbacks, existing structure footprint, and slope all affect what is buildable. Pull your parcel data and have a preliminary conversation with the City of Bothell Planning Department before you design anything.

Step 2: Clarify your intended use. Long-term rental, short-term rental, multigenerational, personal use, investment for resale. The intended use affects which rules apply, what financing makes sense, and how you should structure the project.

Step 3: Talk to your lender before your builder. Equity, financing terms, and the carrying cost of a 10-14 month project have to be part of the feasibility conversation. Know your financial picture before your contractor gives you a bid.

Step 4: Get permit-ready plans, not just design concepts. A design you love that cannot get a permit is not a design. Bothell has specific requirements. Work with a designer or architect who knows local code, not just general ADU theory.

Step 5: Budget for the full picture. Construction cost, permit fees (capped at 50% of primary unit impact fees by state law, but still real), utility connections, insurance adjustments, and carrying costs. The number that looks good on paper usually has at least two line items people underestimated.

ADU laws in Washington State have become significantly more permissive in the last few years, and Bothell has moved in the same direction. The opportunity to add a legal, income-producing unit to an existing Bothell property is real, and for the right lot and the right owner, it is one of the better value-add moves available in this market. But the gap between knowing the concept and knowing the specific rules is where most people either make money or make expensive mistakes. Know what Bothell allows before you order the lumber.

Frequently Asked Questions

Can I build an ADU on my property in Bothell, WA?

Yes, with conditions. Bothell Municipal Code Section 12.14.135 permits ADUs on residentially zoned lots, and Washington State's HB 1337 requires cities within urban growth areas to allow at least two ADUs per lot. In Bothell, that includes both attached ADUs and detached ADUs, as well as garage conversions. Size is limited: the ADU floor area generally cannot exceed 800 square feet or two-thirds of the primary dwelling, whichever is less, though state law requires cities to allow at least 1,000 square feet, so current rules should be confirmed with the City of Bothell Planning Department. One additional parking space is required, and specific height limits apply to detached units, with a general cap of 25 feet. Lot conditions, setbacks, and existing structure footprint will all affect what is actually buildable on a specific parcel.

What is the difference between an ADU and a DADU in Washington State?

An ADU, or Accessory Dwelling Unit, is the umbrella term for any self-contained secondary living space on a residential lot. A DADU, or Detached Accessory Dwelling Unit, is specifically a standalone structure separate from the primary home, typically built in the backyard with its own entrance, walls, and roof. An AADU, or Attached ADU, is connected to the primary home, such as a basement apartment or converted attached garage. The distinction matters for permitting, construction cost, and how the unit functions, with DADUs generally offering more privacy and commanding stronger rental rates, but costing significantly more to build than attached or conversion options.

Do I have to live on the property to rent out an ADU in Washington State?

No. Washington State law removed the owner-occupancy requirement under HB 1337. As of 2025, per the Washington State Department of Commerce, you are not required to live in either the primary residence or the ADU to rent out both units. This represents a meaningful change from prior rules and opens ADU development to investors who do not plan to occupy the property. Note that Junior ADUs, or JADUs, retain an owner-occupancy requirement under federal guidelines, and some local short-term rental regulations may have additional requirements. Always verify current rules for your specific intended use with the City of Bothell.

Can I use an ADU as an Airbnb in Bothell, WA?

Washington State law does not prohibit short-term rentals from ADUs, but local governments retain the authority to restrict or prohibit them. Bothell has its own short-term rental regulations separate from the ADU code. Before building or purchasing a property with the intent of operating an ADU as a short-term rental in Bothell, verify the current rules directly with the City of Bothell Planning Department and the City's business licensing division. Building an ADU optimized for short-term rental income, then discovering local restrictions limit that use, is an expensive problem that is entirely avoidable with a single phone call before you design anything.

Does adding an ADU increase property value in Bothell?

A permitted, well-executed ADU generally adds to property value in Bothell by expanding the buyer pool and providing income that sophisticated buyers factor into their offer calculations. Investors, house hackers, and buyers seeking multigenerational space are all attracted to properties with a legal secondary unit. An unpermitted ADU, however, creates liability, complicates the sale process, and can require expensive remediation before closing. The word to focus on is permitted. A legal ADU adds value. An unpermitted one adds risk. The cost of doing it right from the beginning is almost always less than the cost of addressing it at the point of sale.

How much does it cost to build a DADU in Washington State?

Per Goodwin Construction's 2025 DADU cost guide, construction costs for detached ADUs in Washington State typically range from $250,000 to $450,000, with a modest 600-square-foot DADU in Western Washington running approximately $150,000 to $250,000 all-in. A 1,000-square-foot unit in a higher-cost market can exceed $300,000. The variables that most significantly affect cost are site conditions including slope and existing utility access, size and finish level, whether you are building new or converting an existing structure, and permit timeline. Budget 10 to 14 months from project start to first rental income for a new detached unit, and factor in permit fees, utility connection costs, and insurance adjustments in addition to construction.

Thinking About Adding an ADU to Your Bothell Property?

Let's start with the lot, the rules, and whether the numbers make sense for your situation. That conversation is free. Getting it wrong is not.

Talk to Aaron
AR
Aaron Robinson

Residential Real Estate Agent  ·  Keller Williams Realty Bothell
License #25032471  ·  Greater Seattle Area

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