Can You Build an ADU in King County? What to Check Before You Start Designing
By Leo Lutskov, CEO of Alta Group Remodeling
An ADU can look simple when you first start thinking about it. You may have enough space behind the house, a rough idea of the size you want and a zoning map that seems to say the project is allowed. The reality becomes clearer once we start looking at the property itself. A setback can affect where the building sits, utilities can change the budget, and a critical area or shoreline condition can reduce the part of the lot that is actually practical to build on. I’m Leo, CEO of Alta Group Remodeling, and when a homeowner comes to us with an ADU idea, I do not want to start by drawing the building. I want to understand what the property will support first.
Washington has made ADUs easier to build in many urban areas. State law requires cities and counties to allow at least two ADUs on qualifying residential lots within urban growth areas, and local governments generally cannot require owner occupancy for ADUs covered by those rules. The law also prevents jurisdictions from setting an ADU maximum below 1,000 square feet where the UGA requirements apply. Those changes opened up a lot of possibilities for homeowners, but they did not make every property equally easy to develop. A lot may legally allow two ADUs and still have limited room for a detached building once we account for the actual site.
What I Want to Know Before We Start Designing
The first thing I want is the property address because King County does not have one universal ADU rule. Seattle has its own code, Eastside cities have theirs, and unincorporated King County follows another set of requirements. Rural properties also work differently from urban properties. Once we know the jurisdiction, we can look at zoning, setbacks, and lot coverage and then see whether anything else on the property may affect the design.
Utilities can be just as important as zoning. A detached ADU has to function as a real dwelling, and the property has to support that use. Sewer or septic conditions can change the project before we have even started choosing finishes. If an existing garage or another structure is being converted, I also want to know what condition it is in because having four walls already standing does not automatically make a conversion inexpensive. Sometimes the existing structure gives us a very good starting point. Sometimes bringing it up to residential standards takes enough work that the advantage is much smaller than the homeowner expected.
Critical areas and shoreline rules can affect the project in the same way. A property near water may need additional review, but shoreline jurisdiction is broader than simply being next to a lake or river. Depending on the location, it can include qualifying marine waters, rivers, streams, associated wetlands, and other regulated areas. King County uses different review paths depending on the project, and its current guidance lists a 120-day processing time for shoreline permits. That kind of issue can materially change the schedule, which is why I want to find it before we tell a homeowner when construction will start.
This is also why I prefer doing feasibility before developing a complete design. I would rather simplify a project early than spend months developing something that does not fit the property well. State law may tell us what a jurisdiction has to allow, but the physical site still determines what makes sense to build.
The Rules Change Depending on Where the Property Is
Inside urban growth areas, Washington law requires cities and counties to allow at least two ADUs on qualifying residential lots, and detached ADUs must also be allowed under the state framework. Rural properties follow a different set of rules. King County’s March 2026 guidance generally allows up to two ADUs on urban lots while limiting rural properties to one, so I would never take an urban rule and assume it applies to a rural parcel.
Size limits also vary. Seattle generally limits an ADU to 1,000 square feet, or 1,200 square feet if the unit has three or more bedrooms. Unincorporated King County generally allows 1,000 square feet of heated floor area and 1,000 square feet of unheated floor area, with some urban configurations able to use unused unheated-floor-area allowance to increase heated space up to 1,500 square feet. Those numbers are useful, but I still do not design from the maximum alone. A property may technically allow a 1,000-square-foot ADU while the better project is smaller because of access or the way the main house already sits on the lot.
Eastside cities need to be checked individually as well. Redmond is a good example because its residential zoning changed through Redmond 2050, with the new structure taking effect in 2025. That means an older article or zoning reference may already be out of date for a project being planned now. A King County address tells me the general location, but it does not tell me which local rules apply. We need the actual jurisdiction and current parcel information.
Owner-occupancy rules are another area where I would avoid broad assumptions. For properties covered by the state UGA rules, cities and counties generally cannot require the owner to live in the main home or the ADU. Rural properties follow a different framework, so we check the parcel rather than applying the urban rule automatically. The same is true for parking and other development standards. State law has removed some barriers, but the local code still matters.
What Can Change the Budget and Timeline
Homeowners often start estimating ADU cost from square footage, but site work can change the budget long before we talk about flooring or countertops. In our early planning, site and utility conditions can add roughly 15% to 30% when the property requires more preparation than expected. A detached ADU usually has more infrastructure to solve because it is functioning as a separate building, while an attached unit or conversion may be able to use more of the existing structure. Finish choices can create another significant difference and may add roughly 20% to 40% depending on how far the homeowner moves beyond a standard specification.
Those percentages are not meant as automatic markups for every project. They are a reminder that two ADUs with similar floor plans can have very different budgets if one sits on a straightforward urban lot and the other requires difficult utility work or additional site review. Unincorporated King County also increased most permit-related fees by about 14% for 2026 and added a $126 screening fee to most permit applications. Permit fees alone will rarely decide whether an ADU makes sense, but they belong in the real project budget.
The same site questions affect the schedule. At Alta, the first feasibility stage often takes around 1 to 2 weeks. Design and planning commonly take around 2 to 4 months, although a straightforward conversion can move faster and a detached ADU with additional review can take longer. Construction for a detached ADU often runs around 4 to 8 months, with another 1 to 3 weeks at the end for remaining details and turnover. From early feasibility through completion, a detached ADU can reasonably take around 7 to 14 months.
I would rather give a homeowner that kind of range and tighten it as we learn more about the property than promise a short schedule that only works if everything goes perfectly. A difficult utility connection or an additional review can affect the project much more than a few extra days spent making design decisions at the beginning.
Sometimes the feasibility work also changes the project itself. A homeowner may come to us wanting an ADU, but once we understand the property, a home addition may make more sense. If the goal is simply more room for the same household, an addition can be a simpler solution because an ADU has to function as an independent dwelling. If the space genuinely needs to operate separately, an ADU becomes much more useful. I would rather make that decision based on how the homeowner wants to use the space and what the property makes practical than force an ADU onto a lot because that was the first idea.
Resale plans can matter too. A permitted ADU does not automatically become a separate parcel that can be sold on its own. Washington law allows some ADUs to be sold through a condominium structure, but that requires a different legal setup, and Seattle does not simply treat an ADU as a parcel that can be split from the main property. If separate resale is part of the homeowner’s plan, I want to know that before we design the project because it can change whether an ADU is actually the right approach.
Expert Conversation: The Stone Magazine and Leo Lutskov
The Stone Magazine (TSM): Leo, Washington has made ADUs much easier to build. Does that mean most homeowners can now assume they can add one?
Leo: I would say the rules are much more favorable, especially in urban growth areas, but I would not jump from that to saying every property is ready for an ADU. The site still matters. You can be allowed two units under the zoning rules and still have a property where fitting even one detached ADU well is difficult.
TSM: What do you check first when somebody calls you about an ADU?
Leo: I want the address. Once I know the jurisdiction, I can start looking at the lot and the rules that actually apply there. I would not spend much time talking about the floor plan before that because the property may change what makes sense.
TSM: Are setbacks usually what cause the biggest problems?
Leo: Sometimes, but not always. Utilities can matter more on one property, while a critical area can be the main issue on another. That is why I do not like using one checklist as if every lot has the same problem. We are trying to find the thing that could really change this particular project.
TSM: Do homeowners often assume a garage conversion will be the inexpensive option?
Leo: Yes, because the structure is already standing, and sometimes that does save money. But we still have to look at what would be required to make it a legal dwelling. If the building needs substantial structural or utility work, the difference between a conversion and new construction can become much smaller.
TSM: How much can utilities affect an ADU budget?
Leo: A lot, depending on the site. If the connections are straightforward, that is one project. If we need extensive work to serve a detached unit, it can change the economics before we have selected a single finish. That is why I do not like giving a fixed ADU price from square footage alone.
TSM: When would you recommend a home addition instead?
Leo: If the homeowner does not really need a separate dwelling and the property makes an ADU unnecessarily complicated. If the goal is simply to create another bedroom or more living space for the same household, an addition may solve the problem more directly.
TSM: Can the owner sell the ADU separately later?
Leo: Not automatically. It normally remains part of the same property unless a different legal structure is created where that is allowed. If separate resale is part of the plan, that is something I want to understand early rather than after the ADU has already been designed.
TSM: How early should someone start if they hope to have a detached ADU completed next year?
Leo: Earlier than they probably think. A detached ADU can take around seven to fourteen months from the first feasibility work through completion. Some projects will move faster, but I would rather leave enough room in the schedule than promise a short timeline before we know what the property requires.
TSM: What is the mistake you most want homeowners to avoid at the beginning?
Leo: Designing too much before checking feasibility. It is much easier to change the project while we are still talking about the site than after somebody has already spent time and money developing a plan that does not fit the property well.
