Do I need a permit for an accessory dwelling unit (ADU) in
Des Moines, Washington?

Des Moines ADU permits — the key details

Des Moines operates under Washington State's ADU-friendly regulatory environment, amplified by the city's own 2019 ordinance. The state of Washington removed many local restrictions on ADUs starting in 2019 (RCW 36.70B.430), and Des Moines has embraced this: the city allows detached ADUs, attached ADUs, and junior ADUs (interior one-bedroom units within the primary dwelling) on single-family lots with minimal restrictions. However, every single unit type still requires a building permit from the City of Des Moines Building Department. There is no exemption based on size, owner-occupancy, or construction type. The city's base permit fee for a residential ADU typically ranges from $500–$1,500 for the permit itself, but plan-review fees, impact fees, and utility connection fees often push the total into the $3,000–$8,000 range. This is substantially lower than the $10,000–$15,000 typical in less-permissive regions because Des Moines has streamlined review and eliminated parking mandate costs.

A unique feature of Des Moines compared to Seattle, Tacoma, and some other Puget Sound jurisdictions: the city explicitly allows owner-builders (property owners acting as their own general contractor) to pull permits for owner-occupied ADUs. This can save 10-15% of project costs by eliminating general contractor overhead. However, you must still pass all inspections — the city does not waive building code compliance. Another Des Moines advantage is the elimination of parking requirements. Many Washington cities (including Seattle) require 0.5-1.0 parking spaces per ADU; Des Moines requires zero. This removes a major design and cost headache: you don't need to show a driveway, hardscape, or off-street space, which streamlines plan review and construction. The city also waives or substantially reduces impact fees for ADUs under 750 square feet — check current fee schedule on the city's website, as this may vary year to year.

Egress (emergency exits) is the most common design issue that delays ADU permits in Des Moines. IRC R310 requires at least one operable window or door on each story of sleeping quarters that meets size and sill-height minimums. For a typical detached ADU bedroom, that means a window with a minimum 5.7-square-foot opening (for an adult to escape in a fire) with a sill no higher than 44 inches above the floor. Bedrooms in above-garage or basement spaces are especially scrutinized because they have limited wall space. The city will require this shown clearly on your floor plan and elevation drawings. Second: utilities. Your ADU must have separate utility connections or legally installed sub-meters for gas, electric, and water. You cannot share a main meter with the primary house — the city requires individual billing capability. This is because ADUs are rental-regulated in Washington, and separate metering is part of the rental licensing framework. If your lot is on a septic system (uncommon in Des Moines proper, more common in unincorporated areas), you'll need to verify your septic system has capacity for an additional dwelling unit; many 1970s-era systems are undersized. Third: foundation design. Des Moines sits in IECC Climate Zone 4C (west of the Cascades) with a 12-inch frost depth. For a detached ADU, you'll need frost-protected shallow foundation (FPSF) design per IRC R403.3, which typically means 12 inches below grade with insulation, or a conventional basement/crawl space. The city's Building Department staff are well-versed in Puget Sound soils (glacial till, dense, low permeability); drainage design is usually straightforward but must be shown on plans.

Setback and lot-size rules are where the state/local overlay matters most. Washington state law (RCW 36.70B.430) requires cities to allow detached ADUs on lots of 5,000 square feet or larger; Des Moines is more generous and allows detached ADUs on 4,000-square-foot lots (per local ordinance). Setbacks for detached ADUs in Des Moines are 5 feet from side and rear lot lines (compared to 10 feet for a typical secondary dwelling in other jurisdictions). Front setback is equal to the primary house. Junior ADUs have no additional setback requirement — they're interior, so setbacks don't apply. Attached ADUs (above garage, or side-by-side) follow the primary house setbacks. These rules mean you can fit a detached ADU on a corner lot in Des Moines much more easily than in Seattle or Bellevue. The city has no owner-occupancy requirement — you can permit and build an ADU for rental from day one. This is critical: many Washington cities still require the owner to live in either the primary house or the ADU; Des Moines does not. This opens the door for investment-focused ADU projects and makes the city attractive to out-of-state and out-of-state-income applicants.

Timeline and inspections in Des Moines typically run 8-12 weeks from permit application to certificate of occupancy (CO). The city does not have a formal shot-clock ordinance like California (which mandates 60 days), but staff aim for 4-5 weeks for plan review on a complete, well-prepared ADU package. Inspections follow the standard sequence: foundation/footing, framing, rough mechanical/electrical/plumbing (MEP), insulation, drywall, final building, rough utility, final utility (separate meter if applicable), and planning sign-off. A final step specific to Des Moines ADUs is confirmation that your rental license application (if rental-intended) is queued with the city — the building CO does not trigger the rental license, but the city will flag it so you know the path forward. The city's online permit portal (accessible via the City of Des Moines website) allows you to submit plans electronically and track inspection scheduling, which speeds up communication compared to in-person submission. Many applicants in the Puget Sound region appreciate this because it reduces the number of trips to city hall.

Three Des Moines accessory dwelling unit (adu) scenarios

Des Moines ADU zoning and the state law override

Washington State's ADU-enabling legislation (RCW 36.70B.430, effective 2019) fundamentally changed local control over ADUs. The state law says cities cannot prohibit ADUs on single-family lots; cannot impose owner-occupancy requirements; cannot require more than one parking space per ADU (and many cities, including Des Moines, have reduced this to zero). Des Moines adopted its local ADU ordinance in 2019 to operationalize state rules and actually went further in a pro-ADU direction. The city's ordinance allows detached ADUs on lots as small as 4,000 sq ft (state minimum is 5,000), allows attached/above-garage ADUs without additional approval, and explicitly permits junior ADUs. Unlike Seattle, which still maintains some restrictions (like limiting ADU height to 35 feet, or requiring ADUs to be no more than 75% of primary house square footage), Des Moines has minimal design restrictions. The practical impact: you can build an ADU in Des Moines with fewer design iterations and faster approval than in Seattle or many other Puget Sound cities.

Setbacks in Des Moines for detached ADUs are 5 feet from side and rear lot lines — tighter than the typical 10-15 feet in non-ADU jurisdictions. This is a huge cost-saver because it means you don't need to demolish or relocate much existing landscaping, and you maximize usable lot space. Lot coverage limits are also generous: Des Moines allows up to 50% lot coverage for combined primary + ADU footprint on single-family residential lots, compared to 35-40% in stricter cities. This means if you have a 5,000-sq-ft lot with a 1,500-sq-ft primary house (30% coverage), you can build an 1,000-sq-ft detached ADU (bringing total to 50%) without lot-coverage variance. No variance = no delay, no public hearing, no additional cost.

Rental licensing is separate from building permits. Once your ADU receives a certificate of occupancy (CO) from the Building Department, you still need to register it with the City of Des Moines as a rental dwelling if you intend to rent it out. This is a licensing process, not a permit process, and it costs roughly $200–$400/year. The city requires proof of rental licensing before you can legally rent the unit. This is important: your ADU can be unpermitted in a neighbor's challenge, but if it's permitted and licensed, you have a legal shield. The licensing process includes an inspection by the rental licensing program (same inspectors as Building, usually) to verify code compliance — in other words, the rental license is a secondary verification that your ADU meets code.

Owner-builder eligibility for ADUs in Des Moines is explicitly allowed for owner-occupied units. If you own the primary residence and you're going to live in the ADU (or rent it out, but you'll retain owner-occupancy of the primary house), you can pull a permit as an owner-builder and manage the construction yourself. This can save 10-15% on general contractor overhead (roughly $25,000–$50,000 on a $250,000 project). However, certain trades (electrical, plumbing, HVAC if ductwork, structural framing if complex) will still require licensed contractors in Washington state. You cannot do all the work yourself — you're managing and doing some of the work, but delegating specialized trades to licensed pros. If you plan to construct an ADU as a pure investment (you don't live in the primary house and you're renting out the ADU from day one), you cannot use the owner-builder permit pathway; you must hire a licensed general contractor.

Climate, foundation, and utilities in Des Moines's Puget Sound context

Des Moines sits in IECC Climate Zone 4C (marine), west of the Cascades, with a 12-inch design frost depth and high annual precipitation (roughly 38 inches/year, concentrated October-April). This climate drives three critical ADU design features. First, foundation design: IRC R403.3 requires frost protection (either frost-protected shallow foundation per R403.3.1.1, or a conventional basement/crawl space going 12 inches below frost depth). Most Des Moines ADU builders opt for FPSF: a shallow footing (as little as 12 inches below finished grade) with 2 inches of rigid foam insulation on the outside of the stem wall, running down to the footing. This keeps the footing warm enough that frost-heave doesn't occur. FPSF costs $15,000–$25,000 less than a traditional crawl space and doesn't require excavation into the wet, dense glacial till. The city's Building Department is accustomed to FPSF designs and approves them quickly.

Soil composition: Des Moines's soils are predominantly glacial till (dense silt, clay, and gravel left by the Puget Sound glacier). Bearing capacity is good (2,000-2,500 psf), so typical 12-inch-wide footings don't need to be oversized. However, percolation rate is low, which means drainage can be an issue on flat lots or after heavy rain. Your ADU foundation plans must show exterior drain-tile (6-inch perforated) at the footing with a daylight outlet or sump pump if daylight isn't available. A typical ADU design includes a 4-inch perimeter drain tile at the exterior footing, sloping to daylight (if the lot slopes away from the house) or to an interior sump pit with a pump (if the lot is flat or slopes toward the house). This costs $2,000–$4,000 extra but is non-negotiable in Des Moines; the city will reject plans without it.

Utilities: Des Moines has municipal water and sewer throughout the city proper (unincorporated King County areas may be on septic, but that's outside city limits). For an ADU, you need separate water and sewer service. The water department (City of Des Moines Public Works) can typically run a second meter off the existing service line at a cost of $1,500–$2,500 plus the cost of trenching from the service point to your ADU. Sewer is similar: a second tap into the municipal main, trenched to your ADU, runs $2,000–$3,500. Electric: Puget Sound Energy (PSE, the regional utility) will install a separate meter for the ADU at a cost of $800–$1,500. The city requires that all three utilities be separately metered or sub-metered — no shared main meters. This is to support rental licensing and to ensure fair cost allocation if the unit is later sold or transferred.

Climate resilience and seasonal timing: Des Moines experiences wet, mild winters (32-50°F) and dry, cool summers (60-75°F). Frost depth means you can't install permanent outdoor MEP (water, sewer) between November and March; it will freeze. Most ADU construction in Des Moines runs April-October for this reason, with plan review and permitting happening during winter. If you break ground in the fall, you'll be doing foundation work in late September-October, then waiting until April to install exterior utilities. This can extend timeline by 4-6 months if not planned carefully. The building timeline targets in Scenario A/B/C above assume spring-summer construction; if you're building October-March, add 6-8 weeks for seasonal delays.

Common questions