ADU Utilities and Site Work in Seattle

Two similar ADUs can have different total costs because the work outside the walls differs. A rear-yard unit may need long trenches, a panel or service change, sewer investigation, grading, retaining, drainage or hand-carried construction through a narrow side yard.

Create one existing-services record

Document:

• electric utility, meter, service rating and panel location;
• available panel information and proposed ADU loads;
• water utility, meter and service information;
• sewer location, cleanouts, route and elevation information;
• gas service if it is part of the design;
• communications and address requirements;
• visible drainage paths and low points;
• known easements and overhead/underground lines; and
• the route from street to proposed unit.

Mark facts as verified, observed or assumed. Do not let an assumption become a fixed-price exclusion silently.

Electrical planning

For City of Seattle property, confirm service requirements with Seattle City Light early. Record the existing service, meter and panel, proposed ADU loads, equipment location and whether utility or customer-side upgrades are expected.

A meter spot is not a total electrical design or cost. Ask who confirms service capacity, transformer or line work, trenching, meter strategy, panels, grounding, temporary power and utility charges.

Other cities may use Puget Sound Energy or a municipal utility. Use the address-specific provider and do not transfer a Seattle City Light answer to another jurisdiction.

Water service

For City of Seattle property, confirm the water-service and side-sewer path with Seattle Public Utilities. The building permit does not automatically price a new or upgraded service, trenching, side-sewer repair, right-of-way work or property restoration.

SDCI also tells ADU applicants to acknowledge the King County sewage treatment capacity charge. Confirm the current charge and billing path for the project rather than treating it as a contractor allowance.

Sewer and plumbing route

Ask for the proposed point of connection, route length, slope/elevation basis, cleanouts, trench depth and restoration. Identify whether the design assumes gravity flow or another solution and who verifies it.

For a garage conversion, include slab trenching and patching. For a detached unit, include work between the building and connection point, plus any work in the main building or public way.

Drainage and grading

The site plan should show existing/proposed grades, roof-water handling, paved areas and discharge points. Do not solve a building-placement problem by directing water toward another structure or property.

Ask whether civil, grading, retaining or geotechnical work is included or separately required. Mark export, import, erosion protection and restoration.

Construction access and logistics

Measure gates, side yards, overhead clearance and slopes. Record areas available for material storage, waste bins, pumping, concrete placement and worker access. Note parking, street or neighbor constraints that require permits or coordination.

Ask the builder to explain the construction method it priced. If its estimate assumes machine access that the site does not have, the total is not reliable.

Quote schedule for external work

Do not accept “standard site work included” without quantities or boundaries.

How utilities affect cost and timeline

Utility applications can run alongside design, but the schedule must show dependencies. A permit-ready building plan does not guarantee that agency or field work is ready for construction.

The timeline guide separates provider-controlled work from agency and owner decisions. The cost guide places utility charges and construction in distinct rows.

When should a specialist be involved?

Bring in the relevant designer, engineer, utility representative, plumber, electrician, surveyor or civil/geotechnical professional when the project requires their judgement. A general site visit should not be presented as every specialist’s approval.

Preserve evidence before trenches are closed

Agree on inspections and records before underground work begins. Photograph routes, depths, bedding, joints, cleanouts, valves and separation before backfill, with measurements tied to fixed site features. Collect test results and agency sign-offs where applicable.

Update the site or record drawings when the installed route differs from the design. This information helps future maintenance and reduces guesswork when the main property changes. Include testing and record-document responsibility in the Seattle Washington ADU cost guide, not as an assumed closeout extra.

Put utility assumptions on one plan

Ask the design and construction teams to mark the proposed electrical, water, sewer, drainage and communications routes on the same current site plan. Note connection points, equipment, trench limits, pavement or landscape removal and restoration. This exposes route conflicts with trees, foundations, retaining walls and construction access before separate trades price different assumptions.

The plan does not replace utility approval or field locating. It gives each bidder a common basis and makes later agency changes easier to trace.