In Lake Oswego, a home addition runs into site rules before design: setbacks, lot coverage, the city tree code, sloped lots and sometimes HOA review. Serden Group checks feasibility first, then designs, permits and builds the addition with one project manager.
The best addition for a Lake Oswego home is often decided by the trees and the slope, not the floor plan. We map setbacks, protected trees and grades before sketching, so the design you fall in love with is one the city will approve and the lot can hold.
Rear bump-outs, kitchen and family room expansions, primary suites, second stories and garage conversions — or an ADU in Lake Oswego when a separate unit fits better.
Common on Lake Grove ranches and Westlake two-stories. Engineering, foundation work, new stair, full reroof.
Kitchen, dining and family room expansions where lot coverage, setbacks and protected trees set the footprint.
Bedroom, ensuite bath and closet added up or back, detailed to match the original house.
Basement, garage and detached units permitted as legal dwellings with their own kitchen and entry.
We check your zone’s setbacks and lot coverage, protected trees, slope and any HOA rules before design, so there are no late surprises.
Plans, elevations and stamped structural drawings, a pre-application conference with the city when the project calls for one, and tree-protection plans.
Foundation, framing, roof tie-in, systems and matched finishes, with tree fencing inspected before the building permit is issued.
Why Homeowners Choose Serden
Local Knowledge · Lake Oswego, OR
These are the site and code questions we answer before design, because they decide what size and shape of addition is realistic.
A home addition in Lake Oswego, OR typically costs $110,000–$200,000 for a bump-out, $200,000–$350,000 for a primary suite and $300,000–$500,000 or more for a second story in 2026. Plan on 2–4 months for design and permits and 4–10 months of construction. Additions need City of Lake Oswego building permits, and trees 6 inches or more in diameter are protected.
Lake Oswego protects trees 6 inches or more in diameter at 4.5 ft, and that often decides where an addition can go.
Each residential zone sets its own setbacks, lot coverage and height limits, and many lots here slope.
Going up saves lot coverage and trees but costs more per square foot.
Some approvals come on top of the city permit.
| Scope | Typical cost | Timeline |
|---|---|---|
| Bump-out or single room | $110K–$200K | 4–6 months build |
| Primary suite addition | $200K–$350K | 5–8 months build |
| Second story | $300K–$500K+ | 7–10 months build |
| Design, engineering and permits | Varies by scope | 2–4 months |
Every addition needs a City of Lake Oswego building permit, with trade permits for plumbing, electrical and mechanical. Plans typically include floor plans, elevations, structural calculations and energy documentation. Larger or more complex projects may need a pre-application conference with planning staff first.
We prepare the submittal, coordinate the arborist and engineer, respond to plan-review comments, and schedule inspections.
From First Addition and Old Town to Lake Grove, Westlake, Palisades and Mountain Park — plus West Linn, Dunthorpe and Tualatin. We know where the tree code, sloped lots and HOA review shape what can be built. Our Lake Oswego office is at 333 S State St, Lake Oswego, OR 97034 — design consultations by appointment.
Free in-home consultation — home additions in Lake Oswego, OR & Clackamas County
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Questions? Call (360) 836-7775
Honest 2026 ranges for finished, permitted additions, including design, engineering and permits. Sloped lots and tree protection can add to site costs; we price those after feasibility.
Ranges are planning figures, not quotes. Your fixed price follows feasibility and a written scope.
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Everything Serden Group builds in Lake Oswego, the same service in nearby cities, and the planning guides clients read before they start.
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