home addition permit requirements across 10 verified Seattle-area cities
Side-by-side: which of Seattle, Bellevue, Kirkland, Redmond, Sammamish, Issaquah, Renton, Kent, Federal Way, and Mercer Island require a permit for a home addition, what triggers one, what it costs, and how long review takes. Verified against each city's own page; sources cited per row.
11 of 11 cities have authoritative rules on file · 11 typically require a permit · 0 often do not.
King County
| Jurisdiction | Permit? | Threshold | Fee | Review days | Source |
|---|---|---|---|---|---|
City of Bellevue city | Yes | 2021 IRC R105.1 requires permits for "construction, enlargement, alteration, repair, movement, removal, conversion, or demolition" of structures. Additions are never exempt under R105.2. Footprint additions trigger zoning compliance (setbacks, lot coverage, height); vertical additions trigger structural review for the existing foundation/wall framing. WSEC R402-R403 applies to new conditioned area. SEPA threshold typically ≥12,000 sqft total floor area (RCW 43.21C). | Plan-review fee scales with valuation per UBC Table 1-A. State surcharge $4.50 per building permit. Trade permits (E/P/M) separate. Apply via https://mybuildingpermit.com. | 30–90 days | Cite verified 2026-06-14 |
City of Federal Way city | Yes | Any increase to building footprint or conditioned floor area requires a building permit. A Certificate of Water Availability with fire-flow calculations is required for additions or any change to the building footprint. | Plan-review fee scales with valuation per UBC Table 1-A. State surcharge $4.50 per building permit. Trade permits (E/P/M) separate. | 30–120 days | Cite verified 2026-06-14 |
City of Issaquah city | Yes | A permit is required for all single-family/duplex new construction and additions; no exemption is published. Setback / form / intensity per IMC 18.404. | Plan-review fee scales with valuation per UBC Table 1-A. State surcharge $4.50 per building permit. Trade permits (E/P/M) separate. Apply via https://mybuildingpermit.com. | 45–120 days | Cite verified 2026-06-14 |
City of Kent city | Yes | A residential building permit is required for most construction, alteration, or repair work — including changing a building's occupancy, structure, or living space. Intake fees are billed when the submittal is deemed complete (pay within 15 days); issuance fees are due before the permit is issued. | Plan-review fee scales with valuation per UBC Table 1-A. State surcharge $4.50 per building permit. Trade permits (E/P/M) separate. Apply via https://mybuildingpermit.com. | 30–120 days | Cite verified 2026-05-29 |
City of Kirkland city | Yes | A building permit is required to construct, enlarge, alter, repair, or change the use of a building. Whole-house ventilation compliance is required for new construction and additions greater than 500 square feet of heated space. | Plan-review fee scales with valuation per UBC Table 1-A. State surcharge $4.50 per building permit. Trade permits (E/P/M) separate. Apply via https://mybuildingpermit.com. | 30–120 days | Cite verified 2026-05-29 |
City of Mercer Island city | Yes | Mercer Island distinguishes additions <500sqft (no Intake Screening required) from additions ≥500sqft (Substantial Residential Project, may require Intake Screening prior to review). | Plan-review fee scales with valuation per UBC Table 1-A. State surcharge $4.50 per building permit. Trade permits (E/P/M) separate. | 30–120 days | Cite verified 2026-06-14 |
City of Redmond city | Yes | Additions, alterations, and remodels require a building permit with a site plan. Garage conversion to living space also requires a permit. All trade (electrical/plumbing/mechanical) and fire sub-permits must be completed and passed before the Building Final. | Plan-review fee scales with valuation per UBC Table 1-A. State surcharge $4.50 per building permit. Trade permits (E/P/M) separate. Apply via https://eplans.redmond.gov. | 30–120 days | Cite verified 2026-06-14 |
City of Renton city | Yes | Additions always require a building permit — they are expressly excluded from "ordinary repairs" (RMC 4-5-060 §105.2.2). Work started without a permit is subject to a special investigation fee equal to twice the permit fee. | Plan-review fee scales with valuation per UBC Table 1-A. State surcharge $4.50 per building permit. Trade permits (E/P/M) separate. Apply via https://permits.rentonwa.gov. | 30–120 days | Cite verified 2026-05-29 |
City of Sammamish city | Yes | Single-family/duplex additions require a building permit (MBP: Building → Single-Family Residential → Structure Addition). Demolitions, pools, retaining walls, and detached structures require separate permits. A Sewer Availability Certificate is required when the project exceeds 50% of the assessed value or adds plumbing fixtures. | Plan-review fee scales with valuation per UBC Table 1-A. State surcharge $4.50 per building permit. Trade permits (E/P/M) separate. Apply via https://mybuildingpermit.com. | 30–120 days | Cite verified 2026-06-14 |
City of Seattle city | Yes | Any additional floor area requires Construction Permit (Addition or Alteration). Plan review required. ECA + lot coverage + setback compliance verified at intake. | Construction Permit (Addition/Alteration). Pay 75% of plan-review + permit fees at application acceptance. See SDCI 2026 Fee Estimator. | 30–120 days | Cite verified 2026-06-14 |
City of Shoreline city | Yes | Garage conversion to ADU: use Residential Additions checklist + ADU Worksheet. SMC 20.40.210 limits ADUs to 1,200 sqft (exception: interior ADU on separate floor sharing a common roof with the primary residence may exceed 1,200 sqft). Two ADUs per lot maximum per SMC 20.40.210(A), subject to max-density in Table 20.50.020(1). Critical-area triggers (steep slopes ≥15%, wetlands, streams) require a Critical Areas Report and preapplication meeting before submittal. | Apply electronically via eTRAKiT (https://www.shorelinewa.gov/government/departments/planning-community-development/permit-center-remote-services/register-for-a-professional-etrakit-account). Plan-review fees scale with valuation per City fee schedule. State Building Code Council surcharge $4.50 per building permit. Mechanical/Plumbing permits accompany the Building Permit and cannot be deferred; Electrical permits go through WA L&I (not the City). | 30–90 days | Cite verified 2026-05-17 |
How this table is built
Each row reflects the most-recently-verified authoritative rule on file for that jurisdiction and project type. “Pending verification” means we have the jurisdiction in our database but haven't yet personally checked the city page for home addition rules — those rows fall back to county or WA-state baselines until verified. Sources link directly to the city or county page we read on the verified-on date. See our data-quality leaderboard for how we keep these current.