Before framing.
Get the dirt right.
Site prep and utility hookup are 18–28% of an ADU build budget in the Puget Sound, and the #1 cause of schedule slip when planned late. This is every site task and utility connection, with current ranges and the city office that runs it.
Six utilities, six clocks
| Utility | Options | Cost | Timing | Authority |
|---|---|---|---|---|
| Domestic water | Tap into existing service · separate ¾" or 1" meter · upsize main | $3,500–$14,000 | Permit + 3–6 weeks after pre-pour inspection | SPU (Seattle), Bellevue Utilities, Cascade Water, Tacoma Water |
| Sanitary sewer | Lateral tie-in to existing house line · independent side-sewer · STEP/grinder where required | $6,500–$22,000 | Side-sewer permit + inspection before backfill | SPU, King Co Wastewater, Pierce Co Public Works |
| Electrical service | Sub-feed from main panel (≤ 100A) · independent meter (split-bus or pedestal) · service upgrade to 200/400A | $3,200–$18,500 | Service order 4–8 weeks · panel swap 1 day | Seattle City Light, PSE, Tacoma Power, Snohomish PUD |
| Natural gas | Extend from existing meter · new dedicated meter · stay all-electric (Seattle code default) | $2,800–$7,500 | PSE service order 6–10 weeks | Puget Sound Energy |
| Stormwater / drainage | Sheet-flow dispersal · drywell · perforated pipe trench · connection to public storm | $2,200–$12,000 | Drainage review with permit (concurrent) | City stormwater code per DOE Western WA Stormwater Manual |
| Internet / data | Pull CAT6 from main residence · separate fiber drop (Ziply, CenturyLink, Comcast) | $0–$1,800 | Coordinated with electrical rough-in | Private ISP |
Ten site prep items
| Task | When required | Cost | Notes |
|---|---|---|---|
| Boundary & topographic survey | Always — needed for site plan and setback verification | $1,800–$3,800 | WA-licensed PLS; required on permit set |
| Geotechnical (soils) report | Slopes > 15%, soft/peat soil, or building near critical areas | $3,500–$7,500 | Required by Seattle SDCI on most DADU permits |
| Arborist report & tree retention plan | Any tree ≥ 6" DBH within 25' of work — Seattle SMC 25.11 | $1,200–$2,800 | ISA-certified arborist; protects exceptional trees |
| Demolition (existing structure) | Garage conversion / replacement build | $8,000–$22,000 | Separate demo permit; asbestos survey if pre-1980 |
| Excavation & grading | All foundations; deeper for daylight basements | $4,500–$28,000 | Depends on depth, haul-off distance, rock encountered |
| Erosion / sediment control (TESC) | Always during construction — DOE Manual | $1,400–$3,800 | Silt fence, construction entrance, catch-basin inserts |
| Foundation system | Always — slab, stem-wall, or pin pile on steep sites | $18,000–$55,000 | Engineering required; pin piles for liquefaction zones |
| Retaining walls / shoring | Sloped sites; cut > 4' requires engineering | $4,500–$28,000 | SE wet-stamp required > 4' or surcharged |
| Driveway / parking pad | Where parking required (not within ½ mi of major transit) | $3,800–$12,000 | Pervious surface often required for stormwater credit |
| Landscape restoration | Post-construction — required by most cities | $2,500–$8,500 | Includes irrigation tie-in and code-required plantings |
City-specific utility rules
Separate water meter required when ADU is rented separately; electric submeter optional
Stormwater Manual Vol III — flow control required > 2,000 sqft new impervious
Separate water meter encouraged; sewer side-sewer fee per ERU
Surface Water Engineering Standards — runoff dispersion or detention
Separate meter required for billing if separately leased > 30 days
KZC 105 — LID required where feasible
Tacoma Water — separate ¾" meter for any DADU; sewer service charge per fixture
Tacoma Surface Water Manual — flow control on > 5,000 sqft sites
Separate water meter required for DADUs; one-time SDC per ERU
RMC 15.24 — Phase II NPDES compliance
Sammamish Plateau Water — separate service; capacity charge applies
SMC 21A.50 — critical-area buffer protection
Where to start each service
- Source ↗Seattle Public Utilities — Side Sewer PermitsSide-sewer permit application & inspection process
- Source ↗Seattle City Light — Service ConnectionResidential service order & timeline
- Source ↗Puget Sound Energy — New ServiceGas + electric new construction setup
- Source ↗DOE Stormwater Manual for Western WAStatewide BMP and flow-control reference
- Source ↗Bellevue Utilities — Connect to ServicesWater and sewer connection process
- Source ↗Tacoma Water — Service InstallationNew service connection fees & timeline
- Source ↗Snohomish PUD — New ServiceBuilders & developers service guide
- Source ↗Seattle SMC 25.11 — Tree Protection CodeTree retention, exceptional trees, replacement