SPEC_C / DES MOINES · AADU · Verified 2026-01-15
Des Moines AADU Builders — Attached ADU Permits, Cost & Layout
Des Moines is a quietly strong ADU market — affordable south King County land, light-rail-adjacent demand, and a Permit Services group that runs predictably. An AADU is a self-contained dwelling carved into the existing house — its own entry, its own kitchen, its own bath, with a fire-rated demising wall between units. Des Moines permits attached adu (aadu) on most single-family lots under its current code. DADUs on flat Pacific Ridge and Woodmont lots, AADUs above garages, and basement conversions in older waterfront homes.
SPEC_C.01 / ZONING — Des Moines, King County
Is a aadu allowed in Des Moines?
Des Moines allows attached ADU (AADU) on single-family lots (up to 1,000 sf), no owner-occupancy requirement, and no off-street parking required near transit.
- Max ADU size
- 1,000 sf (verify against city code)
- Parking
- No off-street parking required within ½-mile of major transit per HB 1337; otherwise city standard parking rules apply.
- Owner-occupancy
- Not required
- Height
- DADU heights commonly 18–24 ft per DMMC Title 18.
- Setbacks
- 5 ft side/rear typical for DADUs.
- Lot coverage
- Lot coverage typically 40% in most SF zones.
- Minimum lot size
- No explicit minimum lot size under HB 1337 alignment.
- Tree retention
- Tree-retention applies above thresholds; bluff parcels often carry mature Douglas-fir and madrone.
- Critical areas
- Coastal bluffs along Marine View Drive carry steep-slope and bluff-stability constraints; airport noise contours apply on the north end.
- Alley access
- Alleys uncommon citywide; most DADUs front- or side-loaded.
Feasibility blockers to map early
- coastal bluff (waterfront)
- airport noise overlay (north end)
- steep slope (Marine View)
- Coastal bluffs along Marine View Drive carry steep-slope and bluff-stability constraints; airport noise contours apply on the north end.
- Tree-retention applies above thresholds; bluff parcels often carry mature Douglas-fir and madrone.
SPEC_C.02 / PERMIT — Des Moines Permit Services
Des Moines aadu permit timeline
Plan review (typical)
8–18 weeks
Des Moines Permit Services standard ADU intake via MyBuildingPermit.
With one correction cycle
10–26 weeks
Includes one typical correction cycle.
Permit difficulty
4/10
Higher = more plan-check + design-review friction.
Documents you'll submit
- Site plan with parcel dimensions, structures, setbacks, and tree retention
- Architectural plans (floor plans, elevations, sections)
- Structural plans stamped by a WA-licensed engineer where required
- WSEC energy compliance worksheet
- Stormwater / drainage review per city threshold
- Utility plan (sewer / water / electric)
- Demising-wall and fire-separation detail between primary and accessory units
- Egress plan showing independent entry for the AADU
Common plan-check corrections
- Setback or lot-coverage encroachment on site plan
- Energy-code U-value / glazing percentage mismatch
- Missing structural calcs for shear walls or beams
- Stormwater BMP not selected or sized for the impervious area
- Tree retention plan missing or critical-root-zone fencing not shown
- Fire-separation detail incomplete between units
- Egress missing for the accessory unit
Inspection sequence
- 01Foundation / footing
- 02Underfloor framing & insulation
- 03Rough-in: framing, mechanical, plumbing, electrical
- 04Insulation & vapor barrier
- 05Drywall / wallboard
- 06Final building, plumbing, mechanical, electrical
- 07Certificate of occupancy / final approval
Why Des Moines reviews can slow
- bluff geotech on waterfront parcels
- noise-mitigation construction near airport
- stormwater BMP sizing
SPEC_C.03 / COST — Des Moines cost tier 4/5
Des Moines aadu cost range
$166k–$350k · typical $239k
Existing-space conversions sit at the low end; new additions at the high end.
What moves your number
- Demising-wall scope
- Existing structural capacity
- Egress / window scope
- Mechanical separation
Utility & site notes for this city
- Highline Water and Midway Sewer common providers; confirm per parcel.
- Panel upgrades typically $4–7k including PSE meter swap.
- Side-sewer reviews routine; older laterals on bluff lots can need replacement.
- Bluff shoring and stepped foundations can add $25–60k on waterfront parcels.
Ranges reflect typical 2026 project budgets and exclude land. Your exact cost depends on site access, foundation, utility upgrades, finish level, and city review depth — confirm with a feasibility study before committing.
SPEC_C.04 / TIMELINE — typical end-to-end ~58 weeks
Des Moines aadu schedule, phase by phase
- 01FeasibilityParcel + zoning + utility verification.1–4 weeks
- 02DesignSchematic → permit-ready set.4–12 weeks
- 03EngineeringStructural runs in parallel with design.2–6 weeks
- 04Permit reviewDes Moines Permit Services standard ADU intake via MyBuildingPermit.8–18 weeks
- 05Corrections & resubmittalCycle depends on correction depth.2–8 weeks
- 06ConstructionDetached scope; conversions sit at the lower end.18–38 weeks
- 07Final sign-offFinal inspections and certificate of occupancy.1–4 weeks
Des Moines-specific delay risks · bluff geotech on waterfront parcels · noise-mitigation construction near airport · stormwater BMP sizing
Weather note · Excavation, foundations, and exterior weatherization sequence best Apr–Oct; framing and finishes continue year-round under temporary weather protection.
SPEC_C.05 / COMPARE — AADU vs the other ADU types in Des Moines
Why pick a aadu over the alternatives in Des Moines?
| vs. | Cost | Speed | Privacy | Resale | Best when |
|---|---|---|---|---|---|
| DADU | Lower — reused shell. | Faster — less new structure. | Lower — shared wall. | Slightly less separation; still a permitted second dwelling. | Lot is tight or the main house already has separate-entry potential. |
| Garage Conversion | Similar — depends on existing-conditions scope. | Similar — both reuse a shell. | Better — usually inside the main house, separate entry. | Higher — counts as living area in the main house. | Existing house has unused basement or side wing. |
| Basement ADU | Slightly higher — AADU often touches more of the main envelope. | Similar. | Comparable. | Comparable. | You want a side or upper-level unit, not just a basement. |
SPEC_C.06 / FIT — Is a aadu right for your Des Moines lot?
Best-fit profile for a Des Moines aadu
Pick an AADU when the existing house has the structural capacity, separate-entry potential, and ceiling height to absorb a second unit. Avoid it when reusing the existing envelope would compromise fire separation, egress, or sound transmission below code.
- Rental income
- Strong for long-term rental, especially when the primary unit will be owner-occupied — shared envelope keeps operating costs low.
- Aging parents / multigen
- Excellent — proximity inside the same building, no weather exposure between units, easier caregiving access.
- Tight lot
- Best of the four on a tight lot — no new footprint, no new setbacks to fight.
- Alley access
- Less relevant — AADU entries usually face the street or side yard, not the alley.
- Lower budget
- Lower baseline than DADU — reused envelope, reused roof, reused foundation.
- Privacy
- Lower than DADU — shared wall, sometimes shared mechanical chase or service entry.
- Speed to occupancy
- Faster than DADU — less weather exposure, less framing, no roofing scope.
- Utilities already nearby
- Strong fit — typically extends existing service rather than running new lines.
SPEC_C.07 / SOURCES — verified .gov / city / state references
Official sources for Des Moines ADU rules
- Des Moines Permit ServicesPermit authority for Des Moines ADUs.
- Des Moines ADU informationCity landing page; ADU rules in DMMC Title 18.
- MyBuildingPermit.comShared permit portal Des Moines participates in.
- Des Moines Municipal CodeDMMC zoning chapters with ADU standards.
- RCW 36.70A.681 — Accessory Dwelling UnitsState law setting the floor for ADU regulations cities must meet.
- HB 1337 (2023) — Bill textAuthoritative text of the 2023 ADU reform: two ADUs per lot, 1,000 sf min, no owner-occupancy, parking limits near transit.
- WA Department of Commerce — ADU resourcesState guidance and model code for cities implementing HB 1337.
- WA State Energy Code (Residential)ADUs must meet WSEC; the residential provisions drive insulation, glazing, and HVAC requirements.
Des Moines AADU FAQ
Does Des Moines allow two ADUs per lot?
Yes — one AADU and one DADU on most single-family lots under HB 1337 alignment.
Go deeper: Read the DADU vs AADU in Puget Sound: which to build (2026) guide
Can I convert my Des Moines basement to an ADU?
Yes if ceiling height, egress, and fire separation can be met. Daylight basements with view orientation convert well here.
Do I need a separate electrical meter for an AADU?
Not in most Puget Sound cities. A single meter is acceptable as long as the panel has capacity, the AADU has dedicated branch circuits, and any required sub-panel is sized correctly. Separate meters are usually only required if you plan to bill the AADU tenant for utilities directly.
How thick does the demising wall have to be?
Most plan reviewers require a 1-hour fire-rated separation between dwelling units, with STC 50 sound transmission. The exact assembly depends on the existing wall — UL-listed assemblies and city-approved alternatives are both accepted, but the listing must be on the drawings.
Go deeper: Read the Eastside deck cost & permits: cedar, IPE, composite (2026) guide
Can the AADU share an entry with the main house?
Most cities require a separate, independent entry for the AADU at grade. A shared vestibule with two interior doors is sometimes acceptable, but a shared interior hallway usually is not.
Go deeper: Read the DADU vs AADU in Puget Sound: which to build (2026) guide
Does an AADU need its own HVAC?
Each dwelling unit must have an independent temperature-control source. A dedicated mini-split or fan-coil for the AADU is the common solution — central forced-air shared across both units is generally not allowed under the current Washington Energy Code.
Other ADU types in Des Moines
Same aadu in nearby cities
SPEC_C.08 / NEXT
Plan your Des Moines aadu the right way
A 45-minute feasibility call confirms zoning, utility, and budget reality before you spend on design. Golden State ADU has been licensed in Washington since 2012 (LIC SEATTI*752N6).
More for Des Moines aadu planning
INTERNAL_LINKS / Deep Map