Build a backyard cottage in Seattle: the project workflow

A backyard cottage in Seattle is a 12 to 24 month project that touches at least three city departments, two utilities, and 8 to 14 separate professionals. This is the workflow most owners run through, in the order most builders recommend. Click any step on the project map to read the detail. Steps stacked side-by-side run in parallel.

1. Phase 0

   Feasibility

   Before you pay anyone, confirm your lot can actually host a DADU. Three free checks that take a couple of weeks and tell you whether to proceed. Run them all in parallel.

   2-4 weeks·Free·3 steps
   1. 3 in parallel
      0.1Confirm DADU is allowed on your lotDrop your address into SDCI's free ADUniverse tool and you'll know in about a minute whether a DADU is allowed on your specific lot.Homeowner1 day · Free0.2Pull a SPU Water Availability inquiryAsk SPU in writing whether the water main on your street can serve a new unit, and on what terms. The inquiry is free and you'll have an answer in 1-3 weeks.Homeowner, Seattle Public Utilities1-3 weeks · Free0.3Walk the lot for ECA, slopes, trees, and polesPull the SDCI ECA map for your address and spend half an hour walking the lot. You're noting trees, slopes, fences, and where the power comes in — the inputs your architect needs.Homeowner1-2 days · Free
2. Phase 1

   Site facts

   Get the real numbers about your specific piece of ground before an architect draws a line. This phase feeds design — doing it the other way around is the most expensive avoidable mistake on the project. All four steps run in parallel.

   3-6 weeks·$6,000-$15,000+·4 steps
   1. 4 in parallel
      1.1Boundary + topographic surveyA WA-licensed surveyor sets the legal lot lines, captures grade, marks visible utilities, and inventories every regulated tree with its drip line. Everything downstream lives on top of these lines.Surveyor2-3 weeks · $3,500-$6,0001.2Arborist tree inventory and protection planAn ISA-certified arborist on the Seattle Tree Service Provider Registry produces the tree inventory and protection plan SDCI requires on the permit submittal.Arborist1-2 weeks · $300-$8001.3Geotechnical report (if any ECA, slope, or known soil issue)A PE geotechnical engineer drills borings or digs test pits and produces the soils report that drives foundation design and slope stability.Geotech eng.3-4 weeks · $3,500-$10,000+1.4Camera-scope the existing side sewerGet a plumber or side-sewer contractor to run a camera down the existing line so you and the GC are pricing a connection off a known condition, not a guess.Plumber, Side-sewer contractor1 day · $150-$350
3. Phase 2

   Design

   Schematic to permit-ready set, built on top of the real survey, real tree map, and real geotech from Phase 1. Your architect or designer coordinates the structural, civil, and energy work — those three engineering steps run in parallel.

   2-4 months·$10,000-$45,000·5 steps
   1. 2.1Engage a designer or architectHire a designer or architect with recent permitted Seattle DADUs (verify them through the SDCI Permit Research Tool) and sign a contract that names deliverables and CA scope.Designer, ArchitectEngagement: 1-2 weeks; output: full phase · $5,000-$30,000+
   2. 2.2Schematic design on real surveyFloor plans, elevations, and sections drawn on top of the real survey, tree map, and geotech — iterated with you before structural and civil engineers join.Designer, Architect3-6 weeks · Included in design fee
   3. 3 in parallel
      2.3Structural engineeringA PE structural engineer designs the foundation (per the geotech), framing, retaining walls, and any large openings or decks — and stamps the structural sheets SDCI requires.Structural eng.2-3 weeks · $2,500-$6,0002.4Civil + stormwater designA PE civil engineer produces the drainage plan and on-site stormwater BMP. Almost every DADU triggers this because the footprint plus walkways and parking pad cross 750 sq ft.Civil eng.2-3 weeks · $2,500-$7,5002.5Energy code compliance packageSeattle Energy Code forms, U-value calcs, heat-pump sizing, and ventilation strategy. Most architects handle this in-house.Energy consultant, Architect1-2 weeks · $500-$1,500
4. Phase 3

   Permits

   Four agencies issue the permits a DADU needs: SDCI, SPU, King County Public Health, and Seattle City Light. They run on independent timelines, so file them in parallel — not in sequence. The first four steps run side-by-side; the fifth is the SDCI correction loop that follows submittal.

   3-6 months wall clock·$4,000-$12,000 in fees·5 steps
   1. 4 in parallel
      3.1Submit SDCI construction permit applicationFile the construction permit through the Seattle Services Portal and book the intake appointment the same day. The next available slot is usually 2-4 months out.Designer, Architect, Homeowner, SDCIIntake 2-4 months out; first review 4-8 weeks after intake · ~$3,500 base fee for a 500 sq ft DADU (2026)3.2Submit Seattle City Light service applicationHave the electrician file the SCL service application alongside the SDCI permit. Processing plus crew scheduling sets the energization date at the end of the build, so file early.Electrician, Seattle City Light4-6 weeks processing + crew scheduling backlog · Connection fee varies by site3.3Apply for SPU side-sewer permitSide-sewer permits moved from SDCI to SPU on October 1, 2025. And as of April 2026, core taps and drainage SDC are invoiced up front.Side-sewer contractor, Seattle Public Utilities2-6 weeks · $280-$750+ permit; core tap and SDC invoiced up front (2026)3.4Apply for King County Public Health plumbing permitPlumbing permits inside Seattle are issued by King County Public Health, not SDCI. Your licensed plumber files this one.Plumber, King County1-3 weeks · Varies by fixture count
   2. 3.5Respond to SDCI correction letters (2-4 cycles typical)Your designer turns SDCI's review comments into revised drawings and a response letter. Each cycle takes 4-6 weeks, and 2-4 cycles is typical for a custom DADU.Designer, Architect, SDCI4-6 weeks per cycle, 2-4 cycles typical · Included in design fee; SDCI hourly $292/hr if review goes long
5. Phase 4

   Bid and contract

   Send the permit-ready set to three to five GCs who've actually finished Seattle DADUs. Verify them, negotiate the contract, sign it, bind insurance, and line up financing before mobilization.

   4-8 weeks·$1,500-$5,500 (insurance + optional attorney)·4 steps
   1. 4.1Solicit bids from 3-5 Seattle-DADU GCsSend your permit-ready plans to three to five GCs who've actually built DADUs in Seattle, and ask them all to bid the same scope so you can compare apples to apples.Homeowner, General contractor3-4 weeks · Free
   2. 4.2Verify GC license, bond, insurance, and Seattle DADU historyPull L&I status (active, bonded, insured), check recent Seattle DADU permits on the SDCI Permit Research Tool, and walk through one finished project. About 30 minutes plus a site visit.Homeowner1-2 days · Free
   3. 4.3Negotiate and sign the GC contractFixed-price is what most owners want. Cost-plus only works with a guaranteed maximum price (GMP) cap and weekly itemized invoices. On contracts over ~$300K, an attorney review pays for itself.Homeowner, General contractor, Attorney1-2 weeks · $1,000-$4,000 if attorney-reviewed
   4. 4.4Bind builder's risk + finalize financing drawsGet the builder's risk policy in place before the GC mobilizes, and sync the lender's draw schedule to the milestones in the GC contract.Insurance, Lender, Homeowner1-2 weeks · $500-$1,500 builder's risk; lender fees vary
6. Phase 5

   Build

   Mobilization to substantial completion. The longest phase by wall clock — inspections and weather both compress and stretch the schedule. The first three steps run in sequence; MEP rough-in and finishes overlap; tests and inspections punctuate the whole phase.

   9-14 months·$400-$700 per sq ft of DADU·6 steps
   1. 5.1Site prep, tree protection, utility trenchingStand up tree protection per the arborist plan and open the trenches for water, sewer, and electric so all three trades land their work in one cycle of trenching and backfill.General contractor, Arborist, Side-sewer contractor, Electrician2-4 weeks · $10,000-$30,000+ trenching alone
   2. 5.2Foundation (pin piles if geotech-required)Conventional spread footings on flat lots; pin or helical piles per the geotech recommendation on ECA, sloped, or known-bad-soils lots. This is when the cottage starts to feel real.General contractor, Structural eng.3-6 weeks · Conventional included in GC bid; +$20K-$60K with piles
   3. 5.3Framing and envelopeWall and roof framing, sheathing, weather-resistive barrier, windows and doors, roofing. By the end of this step, the cottage is dry-in and looks like a building.General contractor1-3 months · Included in GC bid
   4. 5.4MEP rough-in (electrical, plumbing, HVAC)Electricians, plumbers, and HVAC subs install rough wiring, pipes, and ducts inside the walls before insulation goes in. Each trade gets its own rough inspection.Electrician, Plumber, HVAC contractor3-5 weeks · $15K-$35K electrical, $15K-$30K plumbing, $9K-$22K HVAC
   5. 5.5Insulation, drywall, finishesInsulation, drywall, cabinets, flooring, paint, fixtures, and appliances. The longest hands-on stretch — and the place where late finish decisions translate straight into budget creep.General contractor2-3 months · Included in GC bid; finish creep is common
   6. 5.6Required tests + inspections at each stageBlower-door test (≤5 ACH50), duct-leakage test (≤4 CFM25 per 100 sf), and SDCI inspections at foundation, framing, rough-in, insulation, and final. The checkpoints between phases.Energy consultant, SDCI, General contractorOngoing across the build · $550-$1,050 for blower-door + duct tests
7. Phase 6

   Energize and occupy

   Substantial completion to legal occupancy. SCL energization is usually the long pole. Nobody can legally move in until SDCI signs off the final inspection and the CofO issues.

   4-12 weeks·Capacity charge starts: ~$46/mo × 15 yr·3 steps
   1. 6.1Seattle City Light energizationThe electrician confirms readiness with SCL and an SCL crew energizes the new service. How long this takes depends on the backlog and how early you filed the service application.Seattle City Light, Electrician2-8 weeks (sometimes longer) · Connection fee paid earlier
   2. 6.2Final SDCI inspection + Certificate of OccupancyGC requests the final inspection; SDCI walks the building, signs off or issues a punch list, and on clearance issues the Certificate of Occupancy that makes the unit legally habitable.SDCI, General contractor1-3 weeks · Included in permit fee
   3. 6.3Move-in and ongoing operating costs beginTenant or family member moves in. The King County wastewater capacity charge auto-bills (~$46/month × 15 years). If you're renting, RRIO registration applies.Homeowner, King CountyOngoing · ~$46/mo × 15 yr KC capacity charge ≈ $8,300 nominal