Cost Breakdown by Type
Home Addition Costs in Snohomish County — By Addition Type
Every addition type has a different cost profile. These ranges reflect current Snohomish County labor and material costs for mid-grade finishes.
Sunroom Addition
$45,000 – $90,000
200–400 sq ft typical
Three-season or four-season sunroom with foundation, framing, windows, roofline tie-in, electrical, and finishes. Four-season adds full insulation and HVAC extension.
Bedroom Addition
$75,000 – $150,000
200–350 sq ft typical
Full structural addition — foundation, framing, roofline, exterior siding, insulation, drywall, flooring, electrical, and egress window. Includes permit management.
Master Suite Addition
$120,000 – $250,000
400–700 sq ft typical
Bedroom plus full en-suite bathroom — requires plumbing rough-in, full tile work, custom shower, vanity, and all finish work. Most complex single-room addition type.
Family Room / Great Room Addition
$90,000 – $175,000
400–600 sq ft typical
Open living space addition with structural beam work, vaulted ceiling options, large windows, flooring, electrical, and possible fireplace or built-in cabinetry.
Garage Conversion to Living Space (ADU)
$55,000 – $130,000
400–800 sq ft typical
Convert existing attached or detached garage to permitted ADU, in-law suite, or bonus room. More cost-effective than full additions — uses existing foundation and structure.
Second Story Addition
$200,000 – $500,000+
800–1,800 sq ft typical
Full second story or partial second floor addition — the most complex addition type, requiring structural engineering, temporary shoring, complete roofline reconstruction, and full mechanical extension.
Cost Per Square Foot
What Does a Home Addition Cost Per Square Foot in Snohomish County?
In Snohomish County (including Edmonds, Everett, Lynnwood, Mukilteo, and Bothell), home additions typically cost between $250–$450 per square foot for a finished, permitted addition at mid-grade finishes. High-end additions with custom finishes can reach $500–$650/sq ft.
These per-square-foot figures include everything — design, engineering, permits, foundation work, framing, roofline tie-in, exterior siding and windows, all interior finishes, plumbing (where applicable), electrical, HVAC extension, and cleanup. The per-square-foot cost is often higher for smaller additions (under 250 sq ft) because many of the fixed costs (engineering, permits, roofline work) don't scale down proportionally.
A garage-to-ADU conversion is typically more cost-effective than a full structural addition because you're working with an existing foundation and structure. For homeowners who need living space and have a usable garage, conversion is almost always the better value.
Permitting
Permits & Approvals for Snohomish County Home Additions
Every home addition in Snohomish County requires a building permit. Here's what you need to know — and why you don't have to navigate this yourself.
All home additions in Snohomish County require a building permit — no exceptions.
Structural additions (anything that modifies the roofline or exterior walls) require stamped engineering drawings.
Permit fees typically run $3,000–$8,000 for full additions in Snohomish County cities.
ADU/garage conversions may require separate zoning review for accessory dwelling unit approval.
Construction in view corridors, shoreline zones, or HOA areas may require additional review.
We manage the entire permitting process — you don't deal with the permit office.