An attached ADU turns square footage you already own into a complete second home with its own entrance, kitchen and bath. Sky Pro Remodeling handles the parts that decide whether it works at all: ceiling height, egress, electrical capacity and the separation between units.
An attached accessory dwelling unit is a complete, independent home built inside or onto the shell of the house you already have. It needs its own kitchen, bathroom, sleeping space and exterior entrance, but shares at least one wall, floor or ceiling with the primary residence. That shared assembly is the whole difference between this and a detached backyard cottage.
On the Eastside, the common versions are a daylight basement finished as a separate unit, a side or rear addition with its own entry, and a unit inside or above an attached garage. Bellevue’s split-level and daylight-basement stock suits this well, since the lower floor often already has grade-level access on the downhill side. What those houses tend to lack is code ceiling height, egress-sized bedroom windows, and enough electrical service for a second kitchen.
Washington has spent recent years loosening limits on accessory dwelling units, and Bellevue has updated its Land Use Code in step. Zoning is often no longer the hard part. The building is: separation between two dwellings, drainage around a below-grade unit, seismic bracing where you cut a door into a braced wall, and utility capacity.
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
Here is what a complete attached ADU project covers, from first walkthrough to final inspection.
We measure ceiling heights, check the electrical service, locate the side sewer, and confirm what your zoning allows on that lot. This tells you whether the unit you want is buildable, buildable with modifications, or not worth pursuing.
An ADU is a new dwelling unit, so the set is heavier than a normal remodel: floor plans, egress details, separation assemblies, energy compliance and usually a stamped structural sheet. We prepare it, submit it, and handle plan review corrections.
Every attached ADU needs its own exterior door, and bedrooms need windows meeting minimum clear opening and sill height for escape. In basements that means enlarging openings, cutting new ones through a concrete stem wall, or building a compliant window well.
The shared wall, floor or ceiling has to be built as a rated separation, with every wiring, plumbing and duct penetration firestopped. We build it to control sound as well, because households sharing a floor plane notice footsteps first.
A legal ADU requires a full kitchen with permanent cooking facilities and a complete bathroom, not a kitchenette and a wet bar. That covers cabinetry, counters, tile, flooring, trim, doors and lighting.
This includes the electrical load calculation and sub-panel or new service, plumbing tied into the existing side sewer, heating, and the mechanical ventilation Washington requires for a new dwelling unit. Sub-metering gets resolved during design, not after framing.
These choices shape comfort, operating cost and buildability more than any finish selection.
| Option | Best For | Considerations | Relative Cost |
|---|---|---|---|
| Ductless mini-split heat pump | Basement and garage conversions with no duct runs | Wall cassettes are visible, and each needs a line-set route plus an outdoor spot for the condenser | Moderate |
| Ducted mini-split with compact air handler | Additions and above-garage units with soffit or attic space | Needs height you may not have below grade, but heats evenly and runs quieter in the room | Moderate to high |
| Double gypsum on resilient channel | Meeting the required separation while cutting noise between households | Adds thickness and eats ceiling height, and the sound benefit disappears if penetrations are not sealed | Moderate |
| Sub-panel from the existing service | Homes where a load calculation shows capacity for a second kitchen | Simplest path, but power stays on your bill unless a sub-meter is added | Low to moderate |
| Separate electrical service and meter | Rentals where the tenant is billed directly by the utility | Requires utility coordination, a new meter base and exterior work on their schedule | High |
| Luxury vinyl plank over a below-grade slab | Basement conversions where slab moisture is a concern | Needs a flat, dry slab and a vapor retarder, but tolerates below-grade conditions far better than wood | Low to moderate |
final material choice depends on site conditions and a written estimate
One walkthrough answers the questions that decide the project: ceiling height, egress, electrical capacity and drainage. Call (206) 825-2773 to schedule it.
Which configuration fits depends far more on your existing structure and grade than on preference.
The lower floor of a sloped-lot home becomes a separate unit, using the downhill wall for the entrance and main windows. It is the most efficient option here because walls, roof and foundation already exist. The catch is ceiling height, since many mid-century lower floors sit below the current minimum.
The garage becomes living space, with the overhead door replaced by a wall, entry door and windows. Slab, roof and three walls are already there, and the floor sits near grade. The consideration is comfort: garage slabs are usually uninsulated and poured without a vapor barrier.
A new wing is framed onto the house, sharing one wall and otherwise built from the footing up. You get full control of ceiling height, windows and layout, which makes it the answer when the basement cannot work. The consideration is zoning, since an addition consumes setback and lot coverage.
A unit is built over an existing attached garage, reached by an exterior stair to its own landing and door. It keeps the ADU out of your daily living space and usually gets the best light. The consideration is structural, because garage framing and footings were rarely sized to carry a second story.
A smaller unit is carved out of existing conditioned space, often a bedroom suite plus an adjoining room, with a door added to the exterior. Nothing is added to the envelope, so structural work stays minor. The consideration is that shrinking the main house is permanent.
An attached ADU follows a longer and far more front-loaded sequence than a standard remodel.
We measure clear height at the lowest beam or duct, read the panel and existing load, and check grade at every wall that could take an entrance. You get an honest read before anyone spends money on design.
The plan is drawn around fixed constraints: where the entrance can go, where egress windows are possible, and where drains can run with usable fall. Kitchen and bath positions get set early because they dictate the plumbing strategy.
Where work removes bracing, cuts concrete or stacks load over a garage, an engineer sizes headers, shear panels, holdowns and footings for Puget Sound seismic requirements. The set then goes to the city and comes back with corrections, which is routine.
Finishes come out, concrete is cut for openings or plumbing, and footings, beams, shear walls and hardware go in. This is also when drainage problems get fixed at the source rather than covered up.
Framing, plumbing, electrical and mechanical are roughed in and inspected before anything closes, including firestopping at every penetration in the shared assembly. Insulation and the air and vapor control layers follow with their own inspection.
Drywall, flooring, cabinets, tile, trim, lighting and equipment go in, with real attention to lighting because a below-grade unit gets little daylight in winter. The city signs off each permit, then we clear the punch list.
Two attached ADUs of identical size can differ enormously in cost, and these are the reasons.
If the lower floor falls short of minimum ceiling height, gaining it means lowering the slab and underpinning the foundation. That is excavation, concrete and engineering at once, and it can outweigh every finish decision combined.
A second kitchen, water heater and heat pump can push an older service past what a load calculation allows. Upgrading a panel is one cost; a new service or separate meter is a much larger one.
New fixtures are cheap to plumb when they sit near existing stacks and drain downhill to the side sewer. When they land below the sewer invert, you are into trenching the slab or adding an ejector pump.
Cutting a door or wide window into a braced wall means replacing that bracing with engineered shear panels and holdowns. Stacking a unit over a garage means proving the foundation can carry it.
Below-grade units frequently need footing drains, dampproofing, regrading or a sump before finish work is worth doing. Skipping that is the most expensive choice available, because the repair then happens after the unit is finished.
Permit and utility connection fees for a new dwelling unit are real line items. Lots with steep slopes, streams or wetland buffers add critical-area review and often a geotechnical report.
Attached ADU costs vary too widely for a meaningful number online. Accurate pricing requires an on-site inspection of your foundation, electrical service, drainage and existing conditions, followed by a written estimate listing the scope in detail.
An attached ADU is code-heavy, and most of the risk sits in decisions made before construction starts.
Ceiling height, egress, electrical capacity and drain routing get checked on the first visit, because those four decide the project. Learning in week one that a configuration will not work costs nothing next to learning it in plan review.
The shared wall or floor sets ceiling heights, duct routes and wiring paths around it, so we detail it before the layout locks. Adding it to a plan that ignored it costs headroom you cannot spare.
We run the electrical load and confirm the utility path before a design assumes a second kitchen, heat pump and dryer. That avoids discovering that a permitted plan needs a service upgrade nobody budgeted.
Below-grade units in this climate fail from water management, not from the wrong flooring. Grade, gutters, downspout discharge and footing drains get addressed before interior assemblies go in.
Egress dimensions, separation details, ventilation and energy compliance are drawn explicitly rather than left for a reviewer to assume. Cleaner submittals mean fewer correction cycles, and that is where ADU schedules vanish.
Most clients occupy the main residence throughout, so dust containment, noise windows, utility shutoffs and site access get planned rather than improvised. You are told in advance which days are loud.
Yes, it has to be reachable without passing through the primary residence. On a daylight basement the door usually goes on the downhill side where the wall already meets grade, while an above-garage unit needs an exterior stair with a landing and a roof over the door. Some jurisdictions also restrict where that entrance can face.
Not automatically, but measure it honestly first. Measure to the lowest obstruction rather than the joists, because ducts, beams and plumbing often hang well below the framing and those govern. If clear height falls short, you can sometimes reroute ducts into a perimeter soffit; otherwise the option is lowering the slab.
Attached ADUs are commonly built as long-term rentals, and Washington has acted to reduce local barriers to renting them. Owner-occupancy and short-term rental rules have changed recently and differ between Bellevue, Seattle and smaller Eastside cities, so confirm the current rule for your address. Tell your insurer as well.
Longer than most people expect, and generally measured in months rather than weeks. An ADU is a new dwelling unit, so the submittal carries structural, energy and life-safety details a bathroom remodel never touches, and every correction cycle adds time. A complete first submittal is what genuinely speeds it up.
Not necessarily. Many attached ADUs run on a sub-panel fed from the existing service and share water and sewer with the main house, which is the simplest arrangement. You move to a separate service and meter when the tenant should be billed directly, or when a load calculation shows the service is short.
More than you would like unless the shared assembly is built for it, especially where footsteps travel through a floor structure. The separation required for fire safety helps, but real sound control comes from decoupling layers, adding mass, insulating the cavity and sealing gaps around outlets and ducts.
Usually, though not always. An attached unit reuses existing foundation, walls and roof, and utility runs are short, which removes the largest line items of a detached build. That advantage narrows quickly if the slab has to be lowered, the foundation underpinned, or a shear wall rebuilt.
Yes, in the large majority of attached ADU projects. Because the work is contained to one portion of the house, the area can be sealed off with dust containment and separate access while your kitchen and bathrooms stay in service. The real disruptions are concrete cutting and brief planned utility shutoffs.
Bring us your floor plan and your goals for the space, and we will tell you plainly what is buildable on your lot and what it would take. Reach us at (206) 825-2773.
Sky Pro Remodeling works throughout Bellevue and the surrounding Eastside communities, including Kirkland, Redmond, Sammamish, Issaquah, Newcastle, Mercer Island and Bothell, along with Seattle. Because ADU rules, fees and review timelines differ between these jurisdictions, we confirm the requirements for your address before design begins.
Do not see your neighborhood? Call (206) 825-2773 — we regularly work throughout King County.
Most attached ADUs start as a basement, and this covers the layout and light strategies that make a lower level feel like a real home.
Useful background on the wiring, framing and foundation surprises common in the mid-century Eastside houses best suited to attached ADUs.
Worth reading if your lower floor will not work and a side or rear addition is the more realistic path to a second unit.
Send a few details about your home and what you have in mind for the unit. We will follow up to arrange an on-site visit and a written estimate.
Let us know how we can help you!
Sky Pro Remodeling — (206) 825-2773 · Contact us
