Seattle allows a second dwelling on a lot that already has a house. Whether a truck can reach your lot, whether a crane can swing the home into place, and whether the Seattle Department of Construction and Inspections (SDCI) approves it on the first review cycle are separate questions. Wolf builds the home in our factory, and a Seattle set means a longer I-5 run followed by city traffic, parked cars, overhead lines, and a backyard that’s often smaller than it looks in photos. We take these projects by arrangement, and every evaluation starts with your specific address.
These requirements apply within Seattle city limits. Tacoma, Olympia, Bremerton, Everett, Shoreline, unincorporated King County, and Bellevue each have their own permitting office and local rules.
Who reviews the permit: SDCI
Inside the city, SDCI issues building permits, and zoning is in the Seattle Municipal Code. A legally permitted unit inside or attached to the house is an attached accessory dwelling unit (AADU). A legally permitted unit in a separate structure is a DADU (detached accessory dwelling unit), which is the backyard cottage Wolf delivers. SDCI’s public tips (116A for AADUs, 116B for DADUs) walk through the city’s process. Clark County’s ePlans system and Portland’s 800-square-foot cap don’t apply here.
Washington law allows two ADUs on qualifying properties in urban growth areas. The available space, utility capacity, and any protected areas still affect what you can build. Read the Washington ADU laws guide for the statute. Seattle’s building department reviews the site plan, trees, stormwater, and utilities. We coordinate the application and explain what your property needs.
HB 1337 also limits how much off-street parking a city can require for an ADU, and eliminates the requirement within a half-mile of a major transit stop. Seattle’s parking maps and frequent-transit definitions still have to be checked against your lot, so we don’t publish a stall count here. If your rental plan depends on parking, tell us on the evaluation and we’ll point you to the current SDCI materials.
Seattle’s Office of Planning and Community Development runs ADUniverse, which includes a gallery of pre-approved DADU designs that can shorten review when a project matches those plans. A Wolf modular home is a factory-built dwelling with a Washington L&I insignia rather than a city stock plan, so the 2–6 week stock-plan timeline doesn’t apply to a module coming up I-5. SDCI still handles the foundation, placement, and the installation permit.
SDCI reports ADU permits to King County for sewer treatment capacity charges, and you acknowledge those charges during the application. It’s an easy line item to miss if you’ve only priced the house.
How Seattle takes a permit
A new DADU goes through the city’s construction addition/alteration process. Here’s how the city describes it:
- Prepare a site plan and submit a Building & Land Use Pre-Application through the Seattle Services Portal. SDCI confirms the legal description, assigns a record number, and may require a pre-application site visit.
- Schedule an electronic intake appointment in the same portal. City materials have noted the next opening can be weeks out. Upload the full package by the city’s cutoff on the appointment day.
- Review covers whatever disciplines your lot needs: zoning, life safety, and sometimes urban forestry, drainage, or Seattle City Light. Incomplete tree, grading, or utility information adds review cycles.
- Request inspections online. Final inspection approval is what allows someone to live in the unit. A typical DADU like this doesn’t go to a Hearing Examiner appeal.
Portals change, so we file in whatever system SDCI is running when the package is ready. The Applicant Services Center offers coaching if you need it. We still walk the lot, because the portal can’t tell you whether a cedar and a service drop share the only crane path.
Models that fit a Seattle backyard
Wolf doesn’t sell HUD code (the federal standard for manufactured homes) manufactured homes. These are modular dwellings inspected in the factory under Washington L&I’s factory-assembled structures program and placed on a local foundation under a city installation permit.
- Model K (~432 sq ft, 12×36). A real bedroom, living room, and kitchen, when the lot can fit a slightly wider single-wide.
- Model E (~616 sq ft, 14×44, two bedrooms). When the yard, coverage, and delivery path can handle a longer module and you need two bedrooms.
Under HB 1337, Seattle can’t cap an ADU below 1,000 square feet. Coverage, rear-yard structure rules, height, tree protection, and whether we can deliver the home still limit what fits, which often makes a 616-square-foot Model E the largest home that can actually be delivered. Setbacks follow the zone and overlay, and those vary from Ballard to Rainier Valley. We confirm the lot first, then pick the model.
The I-5 delivery shapes the design
Our factory is in Battle Ground, in Southwest Washington near Portland, so a Vancouver set is a local drive. A Seattle set goes up I-5 past Joint Base Lewis-McChord, through Tacoma and the city core, and then onto a residential street that may be 24 feet wide with parking on both sides. We measure:
- The truck route from a state route or arterial to the curb.
- Overhead lines: City Light, communications, and street trees.
- The crane pad or roller path from the curb to the foundation.
- Whether the home is a 12-foot K or a 14-foot E, since each needs a different street-use permit.
- Neighbor fences, retaining walls, and a garage that looks like it leaves room until you measure.
Street-use permits and traffic control are separate from the building permit, even when the building permit is clean. A crane on a one-way street in Ballard is a different packet from a roller set off an alley in Georgetown, and we flag that in the first week, because a DADU that can’t be placed is an expensive problem.
Set day needs a weather window, traffic control if the street requires it, and a crew that can weather-seal the same day. Dual-wide plans add a day to join the two halves, which is why most Seattle infill jobs we look at are single-wides. If the only path is over a steep rear yard with no crane access, we’ll tell you on the evaluation rather than sell you a floor plan that can’t come off the truck.
Neighborhoods
Ballard and Phinney often have alleys that look useful until you measure the turning radius and overhead wires. West Seattle adds bridges, hills, and a different approach off the viaduct replacement corridor. Queen Anne and Magnolia combine steep grades with view easements. Beacon Hill, Columbia City, and the Rainier Valley have smaller houses, mature trees, and lots that can take a cottage if the side yard is wide enough. The Central District and Madison Valley are tight. North Seattle grids can be more suburban until you reach a ravine or a greenbelt overlay.
Environmental critical areas, steep slopes, shoreline, and historic districts all change the drawings, so each neighborhood gets its own checklist. If your mailing address says Seattle but the lot is in unincorporated King County or Shoreline, a different office reviews it.
How people use these homes
Seattle DADUs house parents, adult children moving back, long-term renters, and guests, and some are single-level homes for aging in place. HB 1337 removed owner-occupancy requirements inside UGAs, which is why a rental DADU is an option for so many owners. Short-term rentals fall under separate city rules, so don’t assume Airbnb is allowed.
A “tiny home,” a “granny flat,” and an “ADU” are usually the same dwelling if it has a kitchen and bath, and SDCI treats it as a dwelling. The use is what changes parking requirements and whether King County sewer charges and any rental registration apply.
Trees, stormwater, and electrical
Seattle takes trees seriously, and a buildable rectangle on a site plan can shrink once root protection zones are drawn. Drainage on infill lots can swing the cost. If a 100-amp house has to serve a new dwelling, that’s a question for Seattle City Light early on. Laterals, cleanouts, and the need for a new water meter come up on evaluations more often than people expect.
Washington’s energy code and Seattle’s amendments show up on the drawings. The factory builds to the approved plant set, and local inspectors still check the envelope and mechanical systems they’re responsible for.
L&I in the factory, SDCI on site
Washington regulates modular housing under the state building code, not HUD. L&I approves the plant drawings and inspects the structure while it’s being built, and the insignia on the module is why SDCI doesn’t reopen the walls to check the factory wiring. SDCI inspects everything after the truck arrives: foundation, anchorage, any marriage lines, utility tie-ins, grading, and final occupancy. The two tracks run at the same time, so the home is built indoors at the factory while SDCI review and site work move forward. If someone tells you a “mobile ADU” can skip SDCI, they’re selling a different kind of product.
Rain is the reason we build indoors, and it’s also why a Seattle foundation pour in February takes careful scheduling. Owners usually stay in the main house during the project. Your neighbors will notice the crane, so we give you a heads-up on set-day timing to pass along.
Cost
The model gives us a reliable starting home price. We price trenching, crane access, the foundation, and utility-capacity charges after reviewing the property. Factory construction protects the home from Seattle rain, but SDCI review, street-use permits, and utilities still affect the schedule and total. Financing is often a HELOC, a cash-out refinance, or a construction loan from a lender familiar with factory-built ADUs. See financing.
We only describe Seattle projects that appear in our case studies library. Warranty and punch-list work mean a return trip up I-5, which is why we take Seattle projects when the lot, the access, and the owner’s goals are a good fit. That’s what “by arrangement” means in practice.
What a Seattle evaluation looks at
Bring the address, a rough idea of where the building would sit, and any survey, ADUniverse feasibility search, or prior SDCI record number you have. An address and a fence line are enough to start. We look at:
- Whether the lot is inside Seattle, since SDCI only reviews city lots.
- The zone and overlays, then an on-site check for trees, wires, and slope.
- Water, sewer, and electrical: laterals, panel, meter, and the King County capacity-charge acknowledgment.
- The truck route and the crane or roller path to the pad.
- Whether the use is family, long-term rental, or short-term, because occupancy rules follow that.
- Which Wolf model can physically get there.
After we’ve looked at the lot and City Light service, you get a recommendation and a plan. Start here.
Nearby cities
Shoreline files with Shoreline, and Renton and Bellevue have their own development services. Tacoma is Pierce County’s largest city with its own permit office. Bremerton is in Kitsap County, across the water by ferry or the Narrows. Everett and Marysville are in Snohomish County. If your lot is in one of those cities, their local rules apply instead of SDCI’s.
