An ADU is not a bonus room with a hot plate. To be legal it needs its own kitchen, bathroom, sleeping area, and entrance, and it must satisfy the residential code as a standalone dwelling. That distinction separates a unit that appraises and rents cleanly from a finished space that becomes a problem the day you sell.
Washington opened this category up considerably. The state’s 2023 ADU legislation directed cities inside urban growth areas to allow up to two accessory units per residential lot, removed owner-occupancy requirements, and limited parking mandates near frequent transit. Each jurisdiction folded those rules into its own code on its own schedule, so Bellevue, Kirkland, Redmond, and Seattle read differently in the details. The first serious conversation should be about your parcel, not general rules.
The physical work is where the risk lives. A backyard cottage has a high ratio of exterior wall and roof to interior floor area, so water-management details matter more than on an addition tucked under an existing roofline. Add glacial till that sheds water sideways, slopes that funnel runoff toward the back of the lot, and mature firs and cedars that carry regulatory weight, and site work deserves as much attention as the finishes.
This portfolio set documents two completed detached accessory dwelling units, each finished turnkey and photographed staged. The first is a single-level studio with a vaulted ceiling carried on an exposed painted beam, a linear skylight, and a full kitchenette in white shaker cabinetry with matte black bar pulls, a dark subway tile backsplash and a light stone-look countertop. The second is a taller unit built with an open great room and an iron-railed sleeping loft reached by a switchback stair, with the laundry pair tucked into the space beneath the stringer and open wood shelving above it. Both units received complete interior build-outs: cabinetry and countertops, tiled showers with recessed niches and frameless glass, vanities, plank flooring downstairs and large-format tile in the loft unit, recessed lighting, ductless mini-split heads, and finished stucco exteriors with a covered entry and paver hardscape. The tile work is the standout craftsmanship here, from the mitered mosaic shower pan in the studio bath to the patterned feature wall and hex floor in the loft unit’s shower.
Style: Modern Farmhouse
The right form depends far more on your lot and your goal for the space than on aesthetic preference.
A freestanding small house in the rear yard, usually one story or one-and-a-half, with a full kitchen and bath. Best for rental or multigenerational living, because both households get real acoustic and visual separation. It consumes yard and adds impervious surface that triggers stormwater review.
Reuses an existing garage slab, walls, and roof, so the shell is already standing. Often the fastest route to a legal unit and a strong fit for the 1950s through 1970s ramblers common across Bellevue and Kirkland. Garage slabs were poured uninsulated and pitched toward the door, so floor buildup drives the detailing.
A second story over a detached garage, keeping the yard intact and often capturing better light. Suits lots where the backyard is small, sloped, or full of protected trees. Structurally the most demanding option, since older garage foundations were rarely built to carry a dwelling above.
New square footage on the side or rear of the house with its own exterior door, kitchen, and bath. Efficient because utility runs are short and one wall already exists. Fire separation, sound isolation, and a genuinely independent entry path are what make or break it.
A self-contained unit carved from an existing daylight basement, common in Seattle and in Eastside split-levels. Preserves the yard entirely and reuses foundation and roof. Ceiling height, code-compliant egress windows, and below-grade moisture control are the recurring hurdles.
A small unit created within the current footprint, usually from a bedroom suite, with a compact kitchen and a lockable interior door. The lowest-scope path and a good fit for aging parents or a caregiver. You give up interior space, and the units share more infrastructure than owners expect.
The order matters, because early decisions are the expensive ones to reverse.
We walk the property, find where utilities enter, check grade and drainage, and note protected trees. Whether the unit is for rental income, family, or eventual downsizing changes the layout more than people expect.
We confirm what current code allows on your parcel: setbacks, height, coverage, separation from the main house, entry rules, and any critical area overlay. The output is a realistic footprint, not a hypothetical maximum.
Depending on the site, this can include a boundary and topographic survey, an arborist report, and a geotechnical opinion where slopes or fill soils are involved. Skipping these costs more time in review than it saves.
Plans and elevations get refined until the layout works, then the scope is written down: what is included, what is excluded, what allowances apply. A vague scope is the root of most remodeling disputes.
The set goes to the city, and reviewers return corrections across building, land use, drainage, and sometimes fire. We respond and resubmit. Timelines vary by jurisdiction and season, and a firm permit date promised up front is a guess.
Utility locates, excavation, foundation, framing, roofing, windows, and weather barrier, with inspections at each structural milestone. Getting the building dried in before the wet season is something we schedule backward from.
Electrical, plumbing, and mechanical rough-in, then framing and insulation inspections before anything is covered. Air sealing and energy verification happen here, and it is the last easy moment to move an outlet or fixture.
Drywall, paint, flooring, cabinetry, fixtures, and exterior grading, followed by final inspections and the certificate of occupancy. We walk the unit with you and cover how the heat pump and ventilation are meant to run.
A site visit answers what no online calculator can: where the sewer runs, how much yard the setbacks leave you, and which configuration makes sense. Call (206) 825-2773 to schedule one.
An ADU touches nearly every trade, and the scope generally breaks down along these lines.
Before anything gets drawn we check setbacks, lot coverage, height limits, tree rules, and whether the property is mapped as a steep slope, wetland buffer, or stream corridor. This decides whether a detached unit fits the backyard or the garage is the better candidate.
Plan review wants a complete set: site plan with existing and proposed impervious surface, floor plans, elevations, wall sections, structural details, energy code forms, and drainage. A set that answers the reviewer’s questions the first time is the biggest lever on permit speed.
Excavation, footings, and stormwater management for the new roof and hard surfaces. That often means tightline drains to an approved discharge point or dispersion trenches, since routing runoff onto a neighbor’s lot is both a code problem and a neighbor problem.
Framing engineered for the seismic and wind loads this region requires, then sheathing, a water-resistive barrier, flashing at every penetration, and a vented rainscreen gap behind the siding. Wet-climate cladding needs both a drainage path and a way to dry.
Electrical load calculations, water supply, and the sewer or septic connection, often the least visible and most expensive part of the job. A long side sewer run or a panel upgrade can quietly reshape a budget, so we scope it early.
Washington’s energy code sets a high bar for small new dwellings and pushes toward heat pump heating, better windows, air sealing, and continuous ventilation. From there it is drywall, flooring, cabinetry, fixtures, and the inspections ending in a certificate of occupancy.
A handful of early decisions set both the durability of the unit and a meaningful share of its cost.
| Option | Best For | Considerations | Relative Cost |
|---|---|---|---|
| Slab-on-grade foundation | Flat lots, single-level cottages, step-free entries | Needs sub-slab insulation and vapor barrier; plumbing is fixed once poured | Lower |
| Crawlspace foundation | Sloped sites, easier future service access | Adds stem wall and excavation; needs venting or conditioning to control ground moisture | Moderate |
| Fiber cement lap siding | Most detached ADUs and garage conversions | Stable when wet, holds paint, resists rot; still must go over a drainage gap | Moderate |
| Natural cedar siding | Matching older Northwest homes and wooded lots | Regionally fitting, but needs periodic finishing and grows moss on shaded north walls | Higher |
| Architectural composition shingles | Budget builds and matching an existing roof | Familiar and simple to flash; shorter life than metal and holds moss under tree cover | Lower |
| Standing seam metal roofing | Low-slope forms and heavy tree canopy | Sheds needles and moss, lasts decades; demands precise flashing and skilled installation | Higher |
| Ductless mini-split heat pump | Nearly every ADU in this climate | Heats and cools without ductwork; outdoor unit placement affects noise for both households | Moderate |
Final material choice depends on site conditions and a written estimate.
No two ADUs price the same way, and these variables explain most of the spread between projects.
Reusing a garage or basement shell removes foundation, framing, and roofing from the scope, a substantial share of a detached build. Conversions carry their own surprises but start with the expensive components already paid for.
The run back to the water main, the panel, and especially the sewer is trenching that scales with length and with whatever sits on top of it. Crossing a driveway or a root zone moves the number more than finishes will.
A flat backyard with alley access is a different job than a sloped lot where materials get hand-carried past the house. Slope, poor bearing soils, high groundwater, and tight access all add labor and sometimes engineering.
New roof and hard surface area triggers stormwater review, and the response ranges from simple dispersion trenches to engineered detention or infiltration. Which applies depends on how much impervious area you add and what the soils accept.
Permits, land use review, utility connection charges, and impact fees go to jurisdictions and utility districts, not to the contractor, and they vary by city and sewer district. They belong in the budget from day one.
Stock cabinetry and a modest appliance package versus custom millwork and stone surfaces is a wide range inside identical square footage. Kitchens and baths concentrate cost in a small unit, so this choice has outsized effect.
We do not publish ADU pricing, because any number quoted before someone has walked your lot, checked your panel, and located your sewer line is a guess. Accurate pricing requires an on-site inspection and a written estimate reflecting your actual site conditions, scope, and jurisdiction.
An accessory dwelling is a complete small building, and it rewards a builder who treats it as one.
Design coordination, permitting, site work, structure, utilities, and finish sit under one contract. When the drainage plan, the foundation, and the sewer tie-in are managed by the same team, nobody argues in your yard over whose problem a conflict was.
We assemble drawings knowing what reviewers in these jurisdictions consistently flag, and we handle correction cycles directly. That does not make the process instant, but it removes avoidable round trips.
Flashing at every penetration, drainage gaps behind cladding, sill pans, and grade sloping away from the structure are standard practice, not upgrades. Small buildings here fail at their details long before their materials.
Sometimes the honest answer is that a detached cottage will not fit the setbacks, or a protected cedar rules out the spot you had in mind. Better to hear that at the site visit than after paying for drawings.
Much of the Eastside was built between the 1950s and 1980s, and conversions there surface undersized panels, unvented crawlspaces, and framing never meant to carry living space. We look for that during scoping.
You get a written scope, a schedule with real dependencies marked, and updates when something changes. Living on the property during a build is manageable when you know what happens in your yard next week.
Yes, long-term rental of an accessory dwelling unit is a standard permitted use across the region. Short-term rental is what varies, since several cities regulate stays under thirty days separately, with their own licensing and limits. Confirm your city’s rules before design begins if that is your plan.
No, not in most of this area. Washington’s 2023 ADU legislation prohibited cities inside urban growth areas from imposing owner-occupancy requirements, a real change from older codes that required the owner to occupy one of the two units. Some jurisdictions still carry outdated language, so verify it for your address.
State law requires cities in urban growth areas to allow accessory units of at least 1,000 square feet and detached units of at least twenty-four feet in height, a floor local codes cannot go below. Your real constraint is usually the buildable envelope left after setbacks, coverage limits, and tree restrictions. On a typical Eastside lot, yard geometry decides the footprint long before the size cap does.
Often yes, but it depends on what is under and around it. Garage slabs were poured for vehicles, meaning no insulation, frequently no vapor barrier, and sometimes no perimeter footing able to carry insulated walls. The conversion also has to meet ceiling height, egress, energy, and fire separation requirements, so the honest answer comes after someone inspects the slab edge and framing.
Not always. It depends on your jurisdiction, your water and sewer district, and the capacity of your existing service. Many ADUs run off the main house’s water service and a subpanel fed from the existing panel, provided a load calculation shows headroom. A separate meter simplifies billing a tenant but triggers its own connection charges.
Yes. New construction is assessed and added to the parcel’s value, so an ADU increases what you are taxed on. The increase depends on the unit and how the county assessor values it, and it typically appears on the tax roll after completion. Budget for it alongside insurance, which also needs updating.
Plan for design and permitting to take a significant share of the total timeline, often comparable to construction itself. Design comes first, then plan review, which depends on the jurisdiction’s queue and how many correction cycles the set generates. A garage conversion moves faster than a new detached cottage because it skips foundation and shell work.
Incomplete drainage and stormwater information is the most common cause, followed by tree retention issues and critical area triggers nobody checked for at the outset. Sewer routing also stalls projects, especially on older lots where the line’s location is poorly documented. Nearly all of it is discoverable during feasibility, which is why that phase deserves real attention.
Whether you are considering a backyard cottage, a garage conversion, or a lower-level unit for family, we can walk the property and give you a straight assessment before you spend anything on design.
Useful background on how added square footage gets planned and built, which applies directly to attached ADUs and above-garage units.
Worth reading if you are weighing a lower-level conversion, since layout, light, and egress decide whether a basement unit feels livable.
A practical overview of financing options, which matters because an ADU is usually funded differently than a kitchen or bath project.
Sky Pro Remodeling builds accessory dwelling units and garage conversions throughout Bellevue and the wider Eastside, including Kirkland, Redmond, Sammamish, Issaquah, Bothell, and Mercer Island, along with Seattle and neighboring King County communities. Because ADU rules and utility districts change city to city, we work from your jurisdiction’s current requirements rather than a regional template.
Do not see your neighborhood? Call (206) 825-2773 — we regularly work throughout King County.
Tell us about your property and what you want the second unit to do. We will follow up to arrange a site visit and a written estimate based on your actual conditions.
Let us know how we can help you!
Sky Pro Remodeling — (206) 825-2773 · Contact us
