SERVICE ยท ADU Construction
TYPICAL TIMELINE
4โ6 weeks
INVESTMENT RANGE
$30k โ $70k+
SCOPE
Design + build
An accessory dwelling unit (ADU) is a self-contained residential unit on a single-family lot, separate from or attached to the primary home. It has its own entrance, kitchen, bathroom, and sleeping area. A detached ADU is also known as a backyard cottage or DADU.
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Washington State passed HB 1337 in 2023, requiring cities and counties in urban growth areas to allow at least two ADUs per single-family lot. Seattle, Bellevue, Kirkland, Redmond, and every other city in our service area now permit ADUs on most residential parcels, opening the door for homeowners who previously thought their lot was too small or their zoning too restrictive.
A well-located ADU in the Seattle metro can generate significant monthly rent, and demand from the region's workforce keeps vacancy low in most neighborhoods. Actual rents vary by city, unit size, and finishes, ask us for current comps in your area.
A private unit on the same lot, for adult children, aging parents, or in-laws, without buying a second property or giving up shared space in the primary home.
A permitted, completed ADU is a real asset addition. The exact value it adds depends on your neighborhood and comparable sales, we can walk you through local numbers during a consultation.
Rent it now, use it as a home office or guest suite later. A legal ADU is a permanent asset that serves whatever your life requires at the time.
A freestanding structure built in the backyard of the primary lot. This is the highest-value ADU type and provides full privacy between the two units.
An interior conversion carves a self-contained unit out of the existing home, most often a large bedroom, an attached garage, or a basement area, with its own exterior entrance added.
Converting an existing attached or detached garage into a livable ADU. The existing structure (slab, framing, roof) significantly reduces construction cost versus building new.
Expanding the existing home's footprint: a new bedroom wing, in-law suite, home office, or family room attached to the current structure. Not technically an ADU (no separate kitchen required), but built under a similar permit process.
ADU permitting in the greater Seattle area runs through the building department of whichever city you live in, not King County directly. Requirements vary by jurisdiction:
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Statewide rules that apply everywhere (HB 1337, effective 2023, with full city compliance required by 2025):
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Common requirements across all jurisdictions:
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As your licensed general contractor, we pull all permits and manage the full review cycle. Permit liability sits with us, not with you.
A detached ADU requires structural engineering drawings, an energy compliance report, a site plan demonstrating setback and lot coverage compliance, and separate permits for plumbing, electrical, and mechanical. In cities like Seattle, the permit cycle for a DADU regularly runs 10 to 16 weeks, and most first-round submissions come back with comments.
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When your architect and general contractor are different companies, you’re the coordinator between them during the permit cycle. When the reviewer comes back with a comment about the structural connection detail, you’re the one managing the conversation between two firms that are each billing by the hour.
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Renovision’s design-build model puts all of that under one contract:
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One contract. One timeline. One point of contact from first conversation to final inspection.
A detached ADU requires structural engineering drawings, an energy compliance report, a site plan demonstrating setback and lot coverage compliance, and separate permits for plumbing, electrical, and mechanical. In cities like Seattle, the permit cycle for a DADU regularly runs 10 to 16 weeks, and most first-round submissions come back with comments.
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When your architect and general contractor are different companies, you’re the coordinator between them during the permit cycle. When the reviewer comes back with a comment about the structural connection detail, you’re the one managing the conversation between two firms that are each billing by the hour.
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One contract. One timeline. One point of contact from first conversation to final inspection.
Architectural drawings, structural engineering coordination, and energy compliance documentation, produced by our design team with permit approval in mind
We submit to the relevant city building department, manage the review cycle, respond to comments, and track status to approval
The same in-house team that drew the plans builds the unit. What we designed is what you get.
Before a single permit is filed, you approve a photorealistic 3D render of the finished ADU, exterior appearance, interior layout, and materials. No surprises at framing.
2026 COST GUIDE
| Project Type | Typical Range |
|---|---|
| Interior ADU conversion Existing bedroom or basement, own entrance | $60,000โ$120,000 |
| Garage conversion | $90,000โ$160,000 |
| Detached ADU (DADU) New backyard cottage, ground-up | $180,000โ$320,000+ |
| Home addition Attached, expands existing footprint | $150,000โ$350,000+ |
These ranges reflect current King County labor and materials costs and should be revisited periodically as pricing shifts. The biggest cost variables are lot slope and soil conditions (steep or soft lots need additional foundation engineering), finish level (standard versus luxury spec), and structural requirements. Permit fees vary by city; Seattle SDCI and Bellevue Development Services both charge based on project valuation, so ask us for a fee estimate specific to your address.
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If you have received ADU bids significantly below these ranges, ask whether the contractor is licensed, pulling all required permits, and providing structural engineering. Unpermitted ADUs cannot be legally rented, complicate home sales, and may require expensive demolition or legalization to remedy.
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