ADU & DADU Design-Build in Bellevue & the Eastside
An ADU is a second, complete dwelling on a single-family lot, either attached to the house or detached as a backyard cottage (DADU). ECD handles zoning, design, engineering, utilities, permits and construction under one contract. Eastside conversions typically cost $120,000–$260,000; new detached units run $300,000–$600,000 depending on size and site.
Reviewed by Daniel R. Whitcomb, PE · Built by Marcus T. Alvarado
Attached, converted or detached: which ADU suits your property?
The cheapest ADU is space you already have. A daylight basement with its own entry, a garage with a slab in good shape or a lower floor with a separate stair can become a unit for a fraction of the cost of building new, because the foundation, roof and much of the shell already exist. The trade-off is less control over layout and ceiling height, and fire separation work between the units.
A detached unit costs more but gives both households privacy, natural light on all sides and the most flexibility in design. It needs a buildable spot in the yard that meets setbacks and separation distances, a route for sewer and water, and often a path for construction equipment past the house.
| ADU type | Typical size | Main cost drivers |
|---|---|---|
| Basement conversion | 500–900 sq ft | Egress windows, ceiling height, fire separation, moisture control |
| Garage conversion | 300–600 sq ft | Slab insulation and drainage, new walls in the door opening, plumbing under slab |
| Attached addition ADU | 400–800 sq ft | New foundation and roof, tie-in to the house, separate entry |
| Detached DADU, one story | 400–800 sq ft | Foundation, utilities from the street, site access |
| Detached DADU, two story or over garage | 600–1,000 sq ft | Stair, structure, height limits, privacy toward neighbors |
What does ECD's ADU service cover?
- Lot feasibility: zoning, setbacks, height, utilities, slope and access.
- Design of the unit and site plan, including parking where required.
- Structural engineering, energy code compliance and fire separation details.
- Utility planning: side sewer, water service, electrical capacity and service upgrade if needed.
- Permits and address assignment; coordination with utility districts.
- Construction through final inspection and certificate of occupancy where the city issues one.
What we do not provide
Rental listing, property management and tax or financing advice. We provide the drawings, costs and schedule your lender or tax advisor needs.
What are the steps from idea to move-in?
- Lot feasibility2–3 weeks
Zoning, utility routes and buildable area are mapped on your lot.
- Design4–8 weeks
Floor plan, exterior and site plan, with a budget for each option.
- Construction documents and utility plans4–6 weeks
Architectural, structural, energy and utility drawings.
- Permit and utility review8–20 weeks
Building, zoning and utility district reviews run in parallel where possible.
- Site work and foundation3–6 weeks
Excavation, utility trenches, footings and slab or crawlspace.
- Shell, systems and finishes16–26 weeks
Framing to final inspection and occupancy.
How much does an ADU cost on the Eastside?
| Scope | Typical cost | Build time |
|---|---|---|
| Garage conversion, 300–500 sq ft | $120,000–$190,000 | 14–20 weeks |
| Basement conversion, 500–900 sq ft | $140,000–$260,000 | 14–22 weeks |
| Detached DADU, 400–600 sq ft | $300,000–$450,000 | 20–28 weeks |
| Detached DADU, 700–1,000 sq ft | $420,000–$600,000 | 24–32 weeks |
What do state law and Eastside cities require for ADUs?
Washington's housing laws require cities in urban growth areas to allow two ADUs per single-family lot, in attached or detached configurations, and limit owner-occupancy requirements, minimum parking near major transit and impact fees on ADUs. The Washington Department of Commerce publishes guidance for cities. Each city still writes its own size, height, setback and design standards, so a DADU that fits in Kirkland may need changes in Sammamish. Our guide to ADU rules in King County explains how the rules fit together, and each city page notes local constraints.
An ADU we designed and built
PROJECT · SAMMAMISHDetached ADU on a sloped backyardA 640-square-foot one-bedroom cottage with a stepped foundation and a sewer connection routed along the side yard.ADU questions
ADU and DADU FAQ
How much does it cost to build a detached ADU in Bellevue?
A new detached ADU in Bellevue typically costs $300,000 to $450,000 for 400 to 600 square feet and $420,000 to $600,000 for 700 to 1,000 square feet, including design, engineering, permits and utilities on a typical lot. Steep or narrow lots, long sewer runs and high-end finishes push costs higher. Garage and basement conversions cost substantially less.
Can I rent out an ADU on the Eastside?
Yes. ADUs are commonly rented long-term, and state law limits cities' ability to require the owner to live on the property. Short-term rental of an ADU is regulated separately by each city, and some restrict or license it. Check your city's short-term rental rules before planning around nightly rentals.
How large can an ADU be?
Washington law prevents cities from capping ADU size below 1,000 square feet of gross floor area, and some cities allow more. Height, setbacks and lot coverage still apply, so the practical size on your lot may be smaller. Detached units on Eastside lots most often land between 500 and 900 square feet.
Does an ADU need its own utility connections?
Not always. Many ADUs share the house's water service and side sewer, which keeps cost down. Separate electrical metering is common so tenants pay their own power. Some utility districts require a separate water meter or sewer connection for detached units, and a long or uphill sewer route may need a pump. We confirm requirements with your district during feasibility.
Can a garage be converted if I lose the parking?
Usually. State law limits parking requirements for ADUs near major transit stops, and many Eastside cities have reduced or removed ADU parking minimums. Where a city still requires a space for the main house, a driveway pad often satisfies it. We confirm parking rules for your lot before you commit to a conversion.
How long does an ADU permit take?
Plan for 8 to 20 weeks of review on the Eastside. Conversions with no exterior change are at the short end. Detached units with utility, drainage or critical-area review are at the long end. Starting utility district applications at the same time as the building permit is the single biggest schedule saver.
Free estimate · Bellevue office
Talk to the engineer before you commit to a plan
A site visit, a scope conversation and a written budget range, at no cost.