Home Additions in Bellevue & the Eastside
A home addition adds conditioned floor area to the ground floor: a bigger kitchen, a primary suite, a family room or a mudroom. It makes sense when the lot has room within setbacks and lot coverage limits. Eastside ground-floor additions typically cost $400–$650 per square foot, or $60,000 for a bump-out to $350,000 for a full wing.
Reviewed by Daniel R. Whitcomb, PE · Built by Marcus T. Alvarado
Will an addition fit on your lot?
The first question on any addition is not design; it is zoning. Every Eastside city limits how close a building can sit to each property line (setbacks), how much of the lot buildings can cover (lot coverage), how much ground can be paved or roofed (impervious surface) and how tall the house can be. Many lots that look roomy are already close to one of those limits, especially in neighborhoods platted with small side yards.
We check those numbers against county parcel data and a site measurement before drawing anything. If the lot has 300 square feet of allowable coverage left, the design starts from 300 square feet. If coverage is used up, the conversation turns to a second story, a garage conversion or an ADU, which some cities count differently.
Which kind of addition fits what you need?
| Type | Typical size | Common use |
|---|---|---|
| Bump-out | 40–120 sq ft | Kitchen extension, breakfast nook, larger bathroom |
| Single room addition | 150–350 sq ft | Family room, office, bedroom |
| Primary suite | 300–600 sq ft | Bedroom, bath and closet on the main floor |
| Wing addition | 500–1,000 sq ft | Several rooms, often with a new roof line |
| Mudroom or entry | 60–150 sq ft | Storage and entry between garage and house |
What is included in an ECD addition?
- Zoning and feasibility check, measured drawings and design options with budgets.
- Structural engineering for the new foundation, framing, roof and the connection to the existing house.
- Energy code compliance for the new conditioned space.
- Permits, including any drainage, tree or critical-area reviews the city requires.
- Foundation, framing, roofing, windows, siding matched to the existing house, insulation and drywall.
- Electrical, plumbing and HVAC extensions, plus interior finishes to the agreed allowances.
Not included unless scoped
Remodeling of existing rooms beyond the opening into the addition, landscaping restoration beyond the work zone and full re-siding of the house when matching siding is no longer available. We identify each of these at the feasibility stage.
What are the steps and how long do they take?
- Zoning and feasibility1–2 weeks
Setbacks, coverage, height and critical areas are checked against your goals.
- Design and budget3–6 weeks
Layout options, roof and exterior studies, each priced.
- Construction documents3–5 weeks
Architectural, structural and energy sheets for permit.
- Permit review6–16 weeks, by city and site
Zoning, building and sometimes drainage or tree review.
- Foundation through dry-in5–9 weeks
Excavation, footings, framing, roof, windows and siding.
- Systems, finishes and opening into the house8–14 weeks
The existing wall is opened last to keep the house sealed as long as possible.
How much does a home addition cost on the Eastside?
| Scope | Typical cost | Build time |
|---|---|---|
| Bump-out, 60–120 sq ft | $60,000–$120,000 | 8–12 weeks |
| Room addition, 200–350 sq ft | $110,000–$210,000 | 12–18 weeks |
| Primary suite with bath, 350–550 sq ft | $180,000–$340,000 | 16–22 weeks |
| Wing addition, 600–900 sq ft | $260,000–$520,000 | 20–28 weeks |
Which reviews will the city run on an addition?
Expect a building review for structure and energy code, and a zoning review for setbacks, coverage and height. Additions that increase impervious surface may need a drainage plan under the city's stormwater rules, which follow the Washington Department of Ecology stormwater manual. Bellevue and Kirkland also review tree retention; lots near streams, wetlands or steep slopes go through critical-area review. Our Permit Estimator shows typical fees and review times, and each city page lists the reviews that apply locally.
An addition we designed and built
PROJECT · REDMOND · EDUCATION HILLRear bump-out as part of a whole-home remodelA 220-square-foot addition at the back of a two-story house, tied into an existing roof and a new open kitchen.Addition questions
Home addition FAQ
What does a home addition cost per square foot in Bellevue?
Ground-floor additions in Bellevue and nearby Eastside cities usually cost $400 to $650 per square foot with mid-range finishes, including design, engineering and permits. Small bump-outs cost more per foot because foundation, roof tie-in and permit costs are spread over fewer feet. Additions with a kitchen or bathroom sit at the high end because of plumbing and finishes.
How does a new addition connect to the existing roof?
There are three common ways: extend the existing roof plane, add a lower shed roof against the house wall, or build a new gable or hip that ties into the existing ridge. The choice depends on ceiling height, roof pitch and how the addition looks from the street. The engineer designs the framing so new loads do not overload existing rafters.
Can you add onto a house that sits on a crawlspace?
Yes. Most Eastside additions on crawlspace houses use a new perimeter footing and stem wall with a framed floor, matching the existing floor height so there is no step between old and new. The engineer checks the existing footing where the two meet and doweling or a separate footing is detailed depending on soil and load.
Will an addition raise my property taxes?
Usually yes. The King County Assessor revalues a property when new construction adds floor area, and the added value is reflected in the following tax assessment. How much depends on the size and quality of the addition. The Assessor's office can explain how new construction is valued for your parcel.
How close to my property line can I build an addition?
It depends on your zone and lot. Each Eastside city sets front, side and rear setbacks in its land use code, and some use combined side yard minimums or allow reduced setbacks for certain structures. We confirm the exact numbers for your parcel during feasibility, and measure from the actual property line, which sometimes means ordering a survey.
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.