Plumbing is the single most schedule-sensitive trade in an ADU. Framing can flex; a sewer tie-in cannot. This guide covers how rough-in actually runs on Vancouver, Battle Ground, Camas, and Ridgefield ADU projects — what it costs, what gets inspected, and where homeowners lose weeks.
What "rough-in" includes
- Underground / under-slab: drain, waste, and vent (DWV) lines set to grade before concrete.
- Sewer lateral: tie-in to the existing lateral, a new lateral to the main, or a septic connection.
- Water service: supply line from the meter or house manifold to the ADU, buried below frost depth.
- Above-grade DWV: stacks, vents through roof, and branch lines in the stud walls.
- Supply distribution: PEX home runs or trunk-and-branch to each fixture, plus shutoffs.
- Water heater rough: gas line or 240V circuit coordination, T&P discharge, and drain pan routing.
ADU plumbing cost ranges in Clark County
| Scope item | Typical range | Notes |
|---|---|---|
| Rough-in, one-bath ADU | $9,000–$18,000 | Garage conversion near existing stack is lowest |
| New sewer lateral to main | $6,000–$18,000 | Depth, driveway cuts, and street work drive cost |
| Water service trench & line | $2,500–$7,500 | Per 50–100 ft run, hand-dig near trees adds more |
| Separate water meter/tap | $4,000–$12,000 | Utility fees vary by jurisdiction |
| Sewer ejector pump (below grade) | $3,500–$8,000 | Basement ADUs below the main line |
| Finish plumbing & fixtures | $4,000–$10,000 | Toilet, sink, tub/shower, kitchenette |
These are planning ranges from SW Washington projects, not quotes. A firm number comes after a site visit, a camera inspection of the existing lateral, and confirmed utility locations.
Planning an ADU in Clark County?
Send us the site plan or a photo of the existing sewer cleanout and we'll scope the rough-in, trenching, and permit path in writing.
Inspection sequence you have to hit
- Underground/DWV inspection — before backfill or slab pour, usually with a water or air test held on the system.
- Water service inspection — trench open, line pressure-tested, sand bedding visible.
- Rough-in inspection — walls open, all DWV and supply complete, tests still on.
- Gas piping test — if a gas water heater or range is in scope.
- Final plumbing — fixtures set, traps primed, no leaks, water heater strapped and vented.
The mistake that costs weeks: backfilling a trench or pouring a slab before the underground inspection passes. Every jurisdiction in Clark County can require you to re-excavate. Never let a crew cover work that hasn't been signed off.
Old Vancouver homes: what we find
- Galvanized supply in pre-1960 homes — corroded and undersized; tapping an ADU off it starves both units.
- Clay or Orangeburg laterals in older Hazel Dell and Felida neighborhoods — bellied or root-intruded, and not tie-in ready.
- Undersized 3/4" service where a 1" line is needed to serve two dwellings at usable pressure.
- No accessible cleanout, which inspectors will require before final.
Septic vs. sewer ADUs
Rural Clark County parcels on septic need Clark County Public Health approval before an ADU permit is issued. That usually means a system evaluation, a flow-capacity review for the added bedroom count, and sometimes drainfield expansion — a $15,000–$40,000 variable that must be settled at feasibility, not at framing.
Frequently asked questions
Do I need a separate plumbing permit for an ADU in Vancouver, WA?
Yes. In Vancouver and unincorporated Clark County, the ADU building permit does not cover plumbing. A separate plumbing permit is pulled by a Washington-licensed plumbing contractor, with rough-in and final inspections tied to it.
How much does ADU plumbing rough-in cost?
For a one-bath ADU in Clark County, rough-in plumbing typically runs $9,000–$18,000, and $14,000–$30,000 when a new sewer lateral, long trench run, or water-service upsize is required. Garage conversions with a nearby stack sit at the low end.
Can an ADU tie into the existing home's sewer line?
Often yes, if the existing lateral has capacity, adequate slope, and is in sound condition. Most jurisdictions want a camera inspection first. Older Vancouver homes with clay or Orangeburg laterals usually need replacement before a tie-in is approved.
PEX or copper for an ADU water line?
PEX-A with expansion fittings is the common choice in SW Washington — fewer joints, freeze tolerance, and lower labor. Copper still shows up for exposed runs and water-heater connections. Both are code-compliant under the Washington-amended UPC.
Does an ADU need its own water meter?
Not always. Vancouver allows an ADU to share the primary meter in many cases, but a separate meter is required when the ADU is separately billed or the existing service is undersized. Confirm before design — a new meter and service tap adds cost and lead time.
How long does ADU plumbing rough-in take?
Underground and trenching usually take 2–4 days, above-slab rough-in 3–5 days, plus inspection scheduling. Realistically, budget two to three weeks from trench to passed rough-in inspection.
Keep reading
RestorationRepublic coordinates licensed plumbing subcontractors, permits, and inspections under one written ADU scope across Vancouver, Battle Ground, Camas, Ridgefield, and Washougal.
Ready for a written estimate?
Call now or send the details — a project lead responds within 24 hours with a fixed-scope proposal.
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RestorationRepublic handles kitchens, baths, repairs, and water damage across SW Washington — one licensed contractor, one written scope, one accountable point of contact.
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