Woodinville ADU Permit Requirements, in Plain English

The short answer

Can I Build an ADU in Woodinville?

An ADU is an accessory dwelling unit. It is a second, smaller home on the same lot as your house. It can be attached, like a basement apartment or an addition. Or it can be detached, like a backyard cottage. Backyard cottage, in law suite, granny flat, mother in law unit, guest house: if it is a second home on your lot, it is an ADU, and the same rules apply.

Woodinville updated its ADU code in 2025. The old one ADU per lot limit is gone, and so is the owner occupancy rule that older city documents show. Today the code lets you build up to two ADUs on a lot where the city's land use table allows them. You can put one inside your house, attach one, stack one over a garage, or build a separate cottage. This page walks through the current rules and the things that actually shape a project.

First, Make Sure Woodinville Is Your Permit City

This is the question that catches the most Woodinville homeowners. The city says plainly on its own website that a number of Woodinville postal addresses are not inside the city limits. More than half of the 98072 ZIP code sits in unincorporated King County and unincorporated Snohomish County. The 98077 ZIP code is outside the city too, even though the Postal Service calls it Woodinville.

Each of those places has its own rules, its own permit office and its own timeline. Look up your parcel on the city's My Property Info page, which links to the Woodinville Interactive Map and the King County Parcel Viewer. If the City of Woodinville holds your parcel, the rules on this page apply to you.

If Your Address Is in Unincorporated Snohomish County

Many Woodinville mailing addresses north of the city sit in unincorporated Snohomish County, often on rural lots zoned R5 (Rural 5 Acre). The rules there are very different, and they can end a backyard cottage plan before it starts.

Under Snohomish County Code 30.28.010, a new detached ADU is not allowed in the R5 zone on a lot smaller than 5 acres. An attached ADU, such as an addition or a basement or interior conversion, is still allowed on a rural lot with a legal home, and it can be up to 1,200 square feet. So on a one acre R5 lot, the answer is no to a cottage and yes to a unit that shares a wall with the house. Washington's statewide ADU law does not override that county rule on a lot this small. The county does not publish a plan review time, so ask Snohomish County Planning and Development Services before you design anything. Their number is 425-388-3311.

If Your Address Is in Unincorporated King County

Areas such as Cottage Lake and the 98077 ZIP code follow King County code, not Woodinville's. King County has its own ADU limits, its own permit office and its own septic rules. This page does not cover them, and a short call with us will tell you where your lot stands.

Know Your Zone

Woodinville's residential zones are R1, R4, R6, R8, R12 and R18. The number is the target density, in homes per acre. Your zone sets your setbacks, height limit and how much of your lot can be covered by buildings. ADUs are allowed in R1, R4, R6, R8, R12 and R18. The denser R24 and R48 zones and the commercial and industrial zones are not on the ADU list. Check your zone on the city's interactive map before you start to design.

Confirmed rules

Woodinville ADU Rules: What Is Confirmed

Zoning and How Many Units You Can Build

Woodinville allows up to two ADUs on a lot where its land use table lists ADUs as an allowed use. You can mix them any way that works. The second ADU does not count against the density limit for your zone if you build two that meet the ADU rules. Woodinville also has other ways to add homes to a lot, such as a duplex or a unit lot subdivision, and those follow different rules. If you are weighing an ADU against a bigger project on your lot, it is worth asking about both.

Size Limits

Woodinville is generous on size. An ADU must be at least 300 square feet and can be as large as 1,500 square feet of living area. The same range applies to an attached unit and a detached one. The count leaves out parking areas, open air floors and any space with a ceiling under 5 feet. For reference, 750 square feet is a comfortable one bedroom, 1,000 square feet is enough for two bedrooms and one bath, and 1,500 square feet is a full size three bedroom home.

That 1,500 square foot cap is the code's limit, not a promise. Your setbacks, building coverage, trees and utilities decide what actually fits on your lot.

Height Limits

An ADU follows the same height limit as the main house in your zone. That is 35 feet in R1, R4, R6, R8 and R12, and 45 feet in R18. Height is measured from the average finished ground level 6 feet out from the walls. A pitched roof can rise above the limit by up to 10 feet when the extra height is not living space, so ask the city how that applies before you count on it. The code has no lower height cap just for detached ADUs.

Setbacks

A setback is the minimum distance between a building and your property line. Woodinville applies the zone standards to a new ADU, so an attached unit and a detached one follow the same numbers.

If your lot touches a dedicated public alley, a detached ADU can sit right on the property line on the alley side. If you are converting an existing garage or other legal building, it can become an ADU even when it sits closer to the line than today's rules allow, as long as it was built legally in the first place and meets the building code for living space. State law backs this up.

Eaves and some other parts of a building can reach a little past the setback line. A survey and a site plan tell you what really fits.

Building Coverage and Landscaping

Woodinville limits how much of your lot buildings can cover, and the limit depends on your zone and your lot size. The range is wide. On the largest R1 lots, buildings can cover as little as 15 percent. In the denser zones on small lots it can reach 65 percent. The city also sets a minimum amount of landscaping on residential lots, and required landscaping has to stay in place after the ADU, its driveway and its parking are added. The exact numbers are in WMC Table 21.32.050(1). Find your zone and your lot size in that table before you design, or have us run the math.

This is an example, not your number. Coverage counts roofed buildings, so a garage or a covered porch on the plan can use up room you were counting on. A conversion inside an existing building adds no new coverage at all.

Owner Occupancy

Woodinville does not require you to live on the property. The ADU section of the code has no owner occupancy rule, and Washington state law bars cities from requiring one. A 2019 city document shows the old rule, which required the owner to live in one of the units. That rule is gone. You do not have to live on site to build or rent an ADU.

Parking

The ADU section of the Woodinville code does not set its own parking count for an ADU. Parking is handled in the city's general parking rules, and state law limits when a city can require parking for an ADU, including near major transit stops. Because the parking rules are spread across more than one place, ask the city exactly what it requires for your lot before you plan the driveway. If you want a space for the unit, you can always build one.

Renting or Selling

You can rent your ADU long term, and you do not have to live next door. Nightly and weekend rentals are not allowed. The code bars renting an ADU for fewer than 30 consecutive nights. If nightly rentals are part of your plan, they do not work in Woodinville.

On selling, Washington law says a city cannot block a condominium sale of an ADU only because it started as an ADU. Woodinville has also adopted unit lot subdivision rules that let owners split ownership of the units on a lot. Both paths have extra steps and extra investment, so ask the city early if separate ownership is part of your plan. Check your plat and any homeowner association covenants too.

Fees

Here is what the published rules say.

• Park impact fee. An impact fee is a one time charge that helps a city pay for parks, roads and schools as it grows. Woodinville's park impact fee code exempts accessory dwelling units, so a new ADU does not pay the park fee that a new house pays.
• Traffic impact fee. Woodinville's traffic impact fee chapter does not mention ADUs. Ask the city whether the fee applies to your unit and in what amount before you set a budget.
• School impact fee. Washington law that took effect January 1, 2026 says an ADU is not new residential construction for the purpose of school impact fees, even a detached one.
• Building permit and plan review. Woodinville charges building permit fees on a sliding scale based on the value of the work. Plan review is charged on top of that. There is no ADU discount. Ask the permit center for the current fee schedule and an estimate for your project.

The larger investment often comes from utilities. Your water and sewer district charges connection fees, and the power company will quote any new meter or panel work. The utility section below covers who to call.

How You Apply

Woodinville takes building and land use applications online through its Permit Portal, which you can reach from the city's permitting page. The Permit Center is at 17301 133rd Avenue NE and the phone is 425-489-2754. Hours are Monday through Thursday, 7:30 am to 5 pm, and Friday, 7:30 am to 4 pm.

• A new detached ADU, or an addition that becomes an ADU, needs a site plan showing setbacks, building coverage and landscaping, full floor plans and elevations, structural drawings, energy code documents and a drainage submittal sized to the project.
• An ADU created inside your existing house, like a basement conversion, needs floor plans showing the existing and proposed layout, ceiling heights, egress windows and alarm locations. It is a lighter review.

The city publishes no ADU specific checklist or pre approved ADU plans that we could find, so call the Permit Center to confirm what your project needs to include.

Do I Need to Hire an Architect?

No, not by law. Washington state does not require a licensed architect for homes with one to four units, and an ADU falls under that rule. Woodinville does not add an architect requirement of its own. A homeowner, a designer or a design build firm can prepare the drawings. You can read the state rule yourself at RCW 18.08.410.

That said, the drawings have to be right. Drawings that miss what the city asks for come back with comments, and every round of comments adds time. Some parts of the project do need a licensed professional. A geotechnical report must come from a licensed engineer. Structural calculations are needed for anything not sized from the prescriptive code tables.

Watch for these

What Can Complicate Your Project

None of these usually kill a project. They change where the unit goes, what it costs, or how long it takes. Woodinville is generous on size, ownership and park fees. Its tough spots are the city versus county question, trees, sensitive areas and utilities. The sooner you know which ones apply to your lot, the fewer surprises you get later.

City Limits Come First

Everything above only applies inside the City of Woodinville. The most expensive mistake on this page is designing a cottage for a lot that turns out to be in unincorporated Snohomish County, where a detached ADU may not be allowed at all. Confirm the jurisdiction before you pay for a survey or a drawing.

Trees and Roots

Woodinville has its own tree rules, in WMC Chapter 21.50. They cover tree permits, replacement, protection and review. Many Woodinville lots are wooded, and large evergreens can decide where a foundation, a utility trench or a driveway can go. State ADU law says a city cannot hold an ADU to stricter tree retention rules than it applies to the main house, but a tree you want to remove can still need a city permit. Roots matter even when the tree stays. Walk your lot and note anything large before you fall in love with a location.

Streams, Wetlands and Slopes: Critical Areas

Critical areas are places the city treats as sensitive: streams, wetlands, steep slopes and unstable soil. Streams, wetlands and slopes are scattered through the city and the land around it, including along the Sammamish River. Each critical area comes with a buffer, which is a no build zone around it. The city's critical areas code is WMC Chapter 21.51, and the city publishes map layers you can check for free.

A critical area changes the plan in two ways. First, the buffer takes ground you cannot build on. Second, some of the state rules that make ADUs easier to permit do not apply to lots with mapped critical areas or buffers, so the city can apply stricter rules. A stream you have never noticed at the back of your lot can decide where the unit goes.

If your lot has a landslide or erosion hazard, plan for a geotechnical report. A geotechnical report is a soil and slope study done by a licensed engineer. It tells the city whether the ground can safely hold a new building. The city's maps show most hazards before you spend a dollar, but they are a screening tool, not an assessment. Get schedules from local firms before you commit to a start date, because no one publishes a typical turnaround.

Stormwater and Drainage

Stormwater is rain that runs off roofs and paving instead of soaking into the ground. A new ADU adds roof, and often a path or a driveway, so the city will want to see how the runoff is handled. The size of the new hard surface decides how much review you get. A small unit with a short path usually means a lighter review. A larger unit with a new driveway can trigger a full drainage plan. A real site plan with the driveway and walkways drawn in is how you know which review applies. Ask the city for the current stormwater thresholds before you design.

Fire Access and Sprinklers

Fire codes look at how close a fire truck can get. On a deep lot, or one with a long private driveway, a backyard cottage can sit farther from the road than a hose crew can reach. The usual fix is a fire sprinkler system or a wider access path. Sprinklers also change the water meter and backflow device your utility requires, which is covered below. Ask the city's fire reviewer early, because the answer can change the investment in the project by thousands of dollars.

Fire Separation for Attached Units

Once part of your house becomes its own legal home, the two units need a fire rated wall and floor between them. A fire rated assembly is built with materials that slow a fire down, so a fire in one unit cannot spread fast into the other. Any door between the two units has to be a fire rated, self closing door. For a basement ADU, that usually means upgrading the ceiling between the basement and the main floor.

Egress and Basement Conversions

Egress means a safe way out in an emergency. Every bedroom in an ADU needs its own emergency escape window, and the unit needs a clear path to an outside door. In a basement, that often means cutting a larger opening into the foundation and digging a window well outside. This is real construction work, not paperwork.

Basement conversions have a few other checks too:

• Ceiling height. The space needs at least 6 feet 8 inches of ceiling height in habitable rooms. Measure it before you plan anything else.
• A full kitchen. A kitchenette with a sink and a fridge does not count. A legal ADU kitchen needs a permanent cooking appliance, plus the gas or electrical work to run it.
• The separate entrance. A walkout or outdoor stairway still has to be checked against your foundation and your drainage.

Water, Sewer, Septic, Power and Other Utilities

• Water and sewer. Water and sewer service in and around Woodinville comes mostly from Woodinville Water District, but confirm your provider. Under the district's ADU policy, an ADU attached to a single family home can use the water meter and side sewer already permitted for the house. A detached ADU gets its own meter and its own side sewer connection. A three quarter inch meter is the usual size when fire sprinklers are not required. If sprinklers are required, you also install a backflow device behind the district meter. The district's system development charges apply to new connections and to changes to the side sewer for an attached unit. The district issues an availability certificate for new connections, so ask for one early and get the charges in writing before you budget.
• Sewer capacity charge. King County charges a capacity charge for new connections to the regional sewer system. Ask the district whether it applies to your ADU and how it is billed.
• Septic. Lots outside the sewer service area are on septic. A septic system is sized by the number of bedrooms it serves, so a new ADU can force an upgrade or a larger drain field. Health district approval comes before you can submit the permit. Check your system's approved bedroom count first.
• Power. Puget Sound Energy quotes new meters and subpanel feeds by project. Ask for a quote on the feed from your house.
• Gas. Only if you want it. Confirm there is a gas main on your street.
• Address. A separate unit usually needs its own address. Ask the Permit Center how and when to request one.

Timeline

How Long Does a Woodinville ADU Permit Take?

Woodinville has no special review time for ADUs, but it does have a general clock for all permits. After you submit, the city has 28 days to tell you in writing whether your application is complete or what is missing. Once it is complete, the city has 120 days to make its decision. That clock stops while you answer the city's comments or provide a report, so the real time depends on how fast and how completely you respond. The Permit Center can give you a current estimate at 425-489-2754.

Most projects go through more than one round of city comments. A clean, complete first submittal is the single best way to keep that number low.

Here is the order things happen in, and where the delays come from.

1. Jurisdiction and zone check. Confirm the parcel is inside Woodinville city limits, which zone it carries, and who provides water and sewer.
2. Lot review. Your property lines, building coverage, slopes, streams and large trees get measured on the ground, not pulled from records.
3. Design. Unit sizes and locations get tested against the setbacks, the coverage limit, the trees and your yard. You should see options before anything is drawn for the city.
4. Reports, if required. A geotechnical report on lots with a mapped hazard. A tree plan if protected trees are involved. Get real schedules from local firms before you commit to a date.
5. Utility items. These run in parallel with design. The utility availability letter is the one to start earliest.
6. Permit submittal. The application goes in through the city's Permit Portal with the site plan, floor plans, elevations, structural and energy documents, the drainage submittal and the items above.
7. City review. The city reviews the plans and sends comments. The design team answers them and resubmits as needed.
8. Permit issued. Remaining permit fees are paid, and construction can start.

The biggest swing factors in Woodinville are the city limits question, trees and critical areas. A flat lot inside the city with no protected trees and room under the coverage cap can move fast, because there is no owner occupancy rule or park fee to work through. A lot with a stream, a slope or a large evergreen in the way may wait on a redesign, a tree permit or a soil report before the drawings can be locked. Settling those early is the best way to keep your timeline from slipping.