ADU 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 ADU, 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 | Per Bellevue ADU reform (LUC Chapter 20.20.120): ADU size limits were increased from 800 sq ft to 1,200 sq ft or 40% of the structure, whichever is larger. Owner-occupancy is no longer required. No parking is required for ADUs smaller than 1,000 sq ft; one space for ADUs larger than 1,000 sq ft. | Full Construction Permit (Addition/Alteration) fees. Sewer capacity charges may apply (King County: ~$8k-$10k). Apply via https://mybuildingpermit.com. | 21–90 days | Cite verified 2026-06-14 |
City of Federal Way city | Yes | An ADU may be up to 1,000 sq ft of floor area (and may not exceed the square footage of the primary dwelling), with up to two bedrooms, and may be up to 30 feet tall. ADUs are not permitted within required yard setbacks; a minimum 10-foot separation from other structures is required. Three off-street parking spaces are required for a property with an ADU, except within 1/4 mile of a major transit stop. | Full Construction Permit (Addition/Alteration) fees. Sewer capacity charges may apply (King County: ~$8k-$10k). | 30–120 days | Cite verified 2026-06-14 |
City of Issaquah city | Yes | IMC 18.500.090: ADU max 1,000 sq ft GFA excluding non-living areas (garage, storage sheds, decks) — except when the excluded area IS the location of the proposed ADU. Principal unit GFA must be ≥25% greater than largest ADU on the lot. Up to TWO ADUs per lot (any combination attached/detached) allowed alongside the primary residence. Owner- occupancy NOT required (preempted by WA HB 1337). New ADUs not subject to rear-yard setback when rear abuts a public alley (unless the city plows that alley). 4-ft walkway required from ROW/driveway to ADU primary entrance. Height per IMC 18.404 form-and-intensity for the underlying zone (typically 30 ft in SF-D / SF-S / SF-SL). Parking: one additional off-street stall required per Issaquah ADU site-plan rules (may be waived for ADUs within ½ mi of frequent transit per HB 1337). | Building permit fee is valuation-based (Issaquah adopts UBC Table 1-A bracket schedule). Plan review fee 65% of building fee. WA state BCC surcharge $4.50/unit. Application fees invoiced via https://mybuildingpermit.com; 3% credit-card service fee or pay by check / exact cash. Impact fees apply on top of permit fee: traffic concurrency, school, parks. HB 1337 caps ADU impact fees at 50% of single-family unit rate — confirm Issaquah's current adopted ADU schedule at fee schedule URL. King County sewer capacity charge (~$8k-$10k) applies separately if ADU is on the King County sewer system. | 45–120 days | Cite verified 2026-06-14 |
City of Kent city | Yes | Two ADUs per lot are allowed (one attached + one detached, two attached, or two detached) per KCC 15.08.350. A detached ADU shall not exceed 24 feet in height. One additional parking space is required per ADU unless within 1/2 mile of a major transit stop (KCC 15.05.040). Owner-occupancy is no longer required (HB 1337). | Full Construction Permit (Addition/Alteration) fees. Sewer capacity charges may apply (King County: ~$8k-$10k). Apply via https://mybuildingpermit.com. | 30–120 days | Cite verified 2026-05-29 |
City of Kirkland city | Yes | Up to two ADUs per lot (attached and/or detached) are allowed. Owner-occupancy is not required for newly constructed ADUs (HB 1337 / RCW 36.70A.681). One off-street parking space is required unless on-street parking is available within 600 feet or the property is within one-half mile of frequent transit. ADU size/height limits are set in KZC 115.07 — confirm the current adopted figures with Planning (sources conflicted on the exact maximum square footage). | Full Construction Permit (Addition/Alteration) fees. Sewer capacity charges may apply (King County: ~$8k-$10k). Apply via https://mybuildingpermit.com. | 30–120 days | Cite verified 2026-06-14 |
City of Mercer Island city | Yes | MICC 19.02.030 (interim regs implementing WA HB 1337, eff. 6/30/2024): ADU min 220 sqft GFA, max 1,000 sqft GFA AND capped at 25% of the parcel's allowed GFA (whichever is less). Up to 2 ADUs allowed per lot. Owner-occupancy NOT required (preempted by HB 1337). Setbacks: per underlying R-zone (front 20', side 5', rear 25' in R-9.6, etc.; see MICC 19.02.020(B)). Detached ADU may sit at a lot line if the lot line abuts a public alley. Parking: 1 stall/unit for lots <6,000 sf, 2 stalls/unit for lots ≥6,000 sf. Parking waived if parcel is within 1/2 mile WALKING DISTANCE of a light rail station (not generic "major transit"). Separate addressing assigned by City after Land Use approval. HB 1337 caps impact fees at 50% of SFR impact fees. | Phase A Land Use fee per MI Fee Schedule (Type II Land Use Review line). Phase B Building permit fee is valuation-based: ~$2,100 building + ~$1,365 plan review (65%) + WA BCC $4.50 + per-fixture plumbing/mech fees at $280k valuation. Impact fees (school, transportation, parks) ~$5,000-$15,000 BEFORE HB 1337's 50% ADU cap — confirm MI has updated its schedule. | 30–90 days | Cite verified 2026-06-14 |
City of Redmond city | Yes | Redmond ADUs are governed by RZC 21.08.220. A detached ADU is capped at 50% of the combined primary+ADU floor area (excluding garage); the maximum square footage was amended upward under HB 1337 (RZC redline shows 1,000 → 1,500 sq ft) — confirm the currently adopted figure with Planning. Detached ADU height ≤25 ft (≤28 ft within an accessory structure). One additional off-street space is required, waivable within 1/4 mile of a major transit stop. A recorded covenant is required before permit issuance. | Full Construction Permit (Addition/Alteration) fees. Sewer capacity charges may apply (King County: ~$8k-$10k). Apply via https://eplans.redmond.gov. | 30–120 days | Cite verified 2026-06-14 |
City of Renton city | Yes | Renton allows two accessory dwelling units (attached or detached) per legal lot (RMC 4-2-110A, Ord. 6160, 6-16-2025). Onsite owner-occupancy is not required and both units may be rented (HB 1337). Renton retained a 1,000 sq ft ADU maximum; ADU follows the residential-zone development standards (e.g., 5 ft side yard). The building plan-check fee is $0 for ADUs built through Renton's Permit-Ready ADU (PRADU) program. | Full Construction Permit (Addition/Alteration) fees. Sewer capacity charges may apply (King County: ~$8k-$10k). Apply via https://permits.rentonwa.gov. | 30–120 days | Cite verified 2026-05-29 |
City of Sammamish city | Yes | Up to two ADUs per lot are allowed (with a detached single-family residence). A detached ADU can be a maximum of 1,000 square feet; an attached ADU can be up to 50% of the size of the single-family residence or 1,000 square feet, whichever is larger. Detached ADU height is capped at 24 ft. No additional parking is required if the property already has four or more spaces. Owner-occupancy is not required (HB 1337). | Full Construction Permit (Addition/Alteration) fees. Sewer capacity charges may apply (King County: ~$8k-$10k). Apply via https://mybuildingpermit.com. | 30–120 days | Cite verified 2026-06-14 |
City of Seattle city | Yes | Max 2 ADUs per property (Seattle code). Pre-approved DADU plans via ADUniverse cut review to 2-6 weeks. Custom designs follow standard plan submittal (SDCI Tips 103 + 106). Owner-occupancy covenant currently required. | Construction Permit (Addition/Alteration). Pay 75% of plan-review + permit fees at application acceptance. See 2026 Fee Estimator + Director's Rule 5-2025. Pre-approved DADU plans waive most plan-review fees. | 14–84 days | Cite verified 2026-06-14 |
City of Shoreline city | Yes | Max ADU floor area: 1,200 sqft (SMC 20.40.210(C)) with shared-roof/separate-floor exception. Two ADUs per lot max (SMC 20.40.210(A)). Permit pathway: Building Permit required for both new construction and conversion-to-ADU. Garage-to-ADU conversion follows the Residential Additions checklist (per checklist scope statement) + ADU Worksheet. New detached ADU dwelling unit follows the Residential New Build checklist + ADU Worksheet. Reduced impact fees apply for ADUs (vs. primary dwelling). | Apply electronically via eTRAKiT (https://www.shorelinewa.gov/government/departments/planning-community-development/permit-center-remote-services/register-for-a-professional-etrakit-account). Impact fees apply at reduced ADU rates (Fire, Park, Transportation) — paid after permit review, before issuance. State Building Code Council surcharge $4.50. | 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 ADU 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.