How Seattle Handles Accessory Structures: A Practical Guide

Seattle maintains 201 local ordinances across all categories, and 9 of those deal specifically with accessory structures. Here is a breakdown of what the city actually requires, what is prohibited, and where Seattle falls on the strict-to-permissive spectrum compared to other cities.

Tiny Homes

Seattle allows tiny houses on permanent foundations as Detached Accessory Dwelling Units (DADUs) under SMC 23.42.022 and SMC 23.44.041, subject to the same code as any DADU. Tiny houses on wheels (THOWs) are treated as recreational vehicles and may not be used as residences in city residential zones. Sanctioned tiny house villages exist only as transitional encampments under SMC 23.42.054.

Key details: Code Section: SMC 23.42.022, 23.44.041. Foundation: Permanent required (no THOWs). Max Size as DADU: 1,000 sq ft. Max Height: 32 ft (NR/RSL zones). Village Code: SMC 23.42.054.

Living in a tiny house on wheels in a Seattle residential zone is treated as illegal occupancy of an RV/vehicle and is subject to removal and SDCI notice of violation. Unpermitted tiny-house DADU construction triggers a stop-work order and penalties. Transitional encampments operating outside SMC 23.42.054 standards are subject to permit revocation and shutdown.

Carport Rules

Carports in Seattle's Neighborhood Residential zones are regulated as parking/garage accessory structures under SMC 23.44.016, with placement controlled by SMC 23.44.014 (yards). They cannot occupy the front 20 feet of a lot or side yards within 5 feet, and the combined footprint counts toward the 35% lot-coverage limit on lots 5,000 sq ft or larger.

Key details: Code Section: SMC 23.44.016. Front Setback: 20 ft (no parking/carport). Side Setback: 5 ft min. Lot Coverage Cap: 35% (5,000+ sq ft lots). Simple Permit Limit: 750 sq ft, single-story.

Unpermitted carport construction triggers an SDCI stop-work order and notice of violation. First-time parking/setback violations are typically fined $150; additional violations carry penalties of $500. Structures encroaching into required yards must be removed or relocated at owner expense.

ADU Rental Restrictions

Seattle limits short-term rentals (under 30 days) of ADUs to two units per operator under SMC 6.600 — typically the operator's primary residence plus one additional dwelling. ADUs rented short-term must hold a Seattle short-term rental operator license and the rental-platform license. Long-term ADU rentals require RRIO registration under SMC 22.214 but have no minimum term.

Key details: STR Unit Cap: 2 per operator. Operator License: Required ($75/unit/yr). Lodging Tax: 8% (state collected). Long-Term Rental: Allowed, RRIO required. Code Section: SMC 6.600.

Operating a short-term rental without an operator license violates SMC 6.600.090 — civil penalties up to $1,000 per day. Platforms (Airbnb, VRBO) are required to verify licenses and remove unlicensed listings under SMC 6.600.110. RRIO non-registration penalties are $150 per unit per day under SMC 22.214.075. Lodging tax evasion is referred to Department of Revenue.

ADU Owner Occupancy

Seattle eliminated all owner-occupancy requirements for ADUs in 2019 under Ordinance 125854, and this is now reinforced by Washington HB 1337 (codified at RCW 36.70A.696) which prohibits cities from imposing owner-occupancy on ADUs statewide. SMC 23.42.054.B.4 explicitly states no owner-occupancy is required.

Key details: Owner-Occupancy: Not required. Code Section: SMC 23.42.054.B.4. State Law: RCW 36.70A.696. Deed Restriction: None required. Rental Registration: RRIO required.

There is no penalty for non-owner-occupancy of an ADU itself. Failure to register an ADU rental with RRIO under SMC 22.214 is subject to fines of $150 per unit per day. Violations of just-cause eviction rules (SMC 22.206.160) can result in tenant damages of 2-3x rent plus relocation assistance.

Seattle is more permissive than most cities when it comes to adu owner occupancy. That said, there are still limits.

ADU Permits

Seattle permits attached (AADU) and detached (DADU) accessory dwelling units ministerially under SMC 23.42.054 and 23.44.041, with HB 1337 (2023) requiring up to two ADUs per residential lot statewide. Applications are filed with Seattle Department of Construction & Inspections (SDCI) and SDCI must act within 120 days (Type I land-use review).

Key details: Code Section: SMC 23.42.054, 23.44.041. ADUs Per Lot: Up to 2 (HB 1337). Review Type: Type I ministerial. SDCI Review Clock: 120 days max. Pre-Approved Plans: ~30-day review.

Building an ADU without permits triggers an SDCI Notice of Violation under SMC 23.90, daily civil penalties of $150-$500, and double permit fees on application to legalize. Continued non-compliance can result in Director's Orders to remove or restore the structure.

The rules around adu permits in Seattle lean permissive, but that does not mean anything goes.

ADU Impact Fees

Seattle does not charge transportation, school, or park impact fees on ADUs because the city has not adopted general impact fees under RCW 82.02. Permit fees, sewer/water capacity charges from Seattle Public Utilities, and Mandatory Housing Affordability (MHA) fees may apply to larger projects, but standalone ADUs are largely exempt.

Key details: City Impact Fees: None (school/parks/transportation). MHA Fee: Not applicable to ADUs. Permit Fees: ~$3-8K typical. SPU Capacity Charge: Yes, capped for infill. Future Fee Cap: RCW 36.70A.681.

Failure to pay required SPU and King County capacity charges blocks permit issuance and final inspection. Unpaid fees become a lien on the property and accrue interest. Misrepresenting ADU size to qualify for fee reductions violates SMC 23.90 with civil penalties up to $1,000 per occurrence.

Seattle is more permissive than most cities when it comes to adu impact fees. That said, there are still limits.

Shed Rules

One-story detached accessory structures (sheds, playhouses) under 200 sq ft do not require a building permit under Washington State Building Code (IRC). Structures in Environmentally Critical Areas require additional ECA review regardless of size. Setback and zoning requirements apply.

Key details: Permit Threshold: No permit if ≤ 200 sq ft (non-ECA). ECA Exception: ECA review required in critical areas. State Code: WA Building Code (IRC) per RCW 19.27. SDCI: Contact for site-specific guidance: (206) 684-8600.

Oversized unpermitted shed: SDCI violation. Living in a shed: code violation requiring full permit compliance.

The rules around shed rules in Seattle lean permissive, but that does not mean anything goes.

Garage Conversions

Garage conversions to ADUs are allowed in Seattle under SMC 23.42.022 as part of HB 1337 compliance. Conversion provisions apply in all residential zones. Building permits required from SDCI. Must meet Seattle Building Code and accessibility standards.

Key details: Allowed: Yes, per SMC 23.42.022 (June 2025). Permit: Building permit required from SDCI. Code: WA Building Code / SMC 23.42.022. Contact: SDCI: (206) 684-8600.

Unpermitted conversion: SDCI stop-work order. Non-code-compliant habitation: notice of violation and potential fines.

Seattle is more permissive than most cities when it comes to garage conversions. That said, there are still limits.

ADU Rules

Seattle adopted Ordinance 127211 (June 2025) to comply with HB 1337. Up to 2 ADUs allowed per lot by right in all residential zones (SMC 23.42.022). No owner-occupancy requirement. Max ADU size 1,000 sq ft. ADUs cannot be used as STRs. DADUs allowed up to 32 ft in most NR zones.

Key details: Code: SMC 23.42.022 (Ord. 127211, June 2025). State Mandate: HB 1337 (2023) — 2 ADUs per lot required. Owner-Occupancy: Not required. Max Size: 1,000 sq ft. DADU Height: Up to 32 ft in most NR zones.

Unpermitted ADU construction: SDCI stop-work order and penalties. Non-compliant ADU: notice of violation.

The rules around adu rules in Seattle lean permissive, but that does not mean anything goes.

The Bottom Line

Compared to many U.S. cities, Seattle gives residents more room on accessory structures. 6 of the 9 rules here are rated permissive. But permissive does not mean unregulated. There are still requirements, and the city does enforce them when violations are reported.

Keep in mind that Seattle can amend these rules at any council meeting. For the most current version of any rule mentioned here, check the specific ordinance page, where we track updates as they happen.