How do sewer lines work in a Seattle townhome, and what should I inspect?

How townhome sewers are laid out

A row of four townhomes usually has one connection to the city main, not four. Each unit has its own building drain and a short branch line, and those branches join a shared private sewer — typically under the driveway or the courtyard — that runs to the street. The shared section is not the city's; it belongs jointly to the units it serves. Seattle allows this arrangement under its side sewer code, and the arrangement is supposed to be documented in a recorded maintenance agreement or in the HOA's declaration.

What you own and what you share

The reason this matters: a backup in your unit can be caused by a defect in a section you only partly own, and a defect in your branch can back up the neighbor. The shared side sewer article goes into the payment rules in detail.

What to inspect before buying

1. Scope your branch. From the unit's cleanout (townhomes almost always have one) to the junction with the shared line. Look for sags under the driveway, crushed sections, and construction debris.
2. Look at the junction. The camera should show a clean wye into the shared line, not a hammered-in tap.
3. Ask to scope the shared line. It needs access from a shared cleanout or a neighbor's; a written request through the HOA or the neighbors usually works, and a buyer with a scheduled inspection has a good reason to ask.
4. Read the agreement. Find the recorded maintenance agreement or the relevant section of the declaration. It should name the shared line, say how costs are split, and say who can authorise a repair. If there is no agreement, that is a finding in itself.
5. Pull the permit history. the city's side sewer permit records (SDCI before October 2025, Seattle Public Utilities since) show the original permit and any repairs.

What goes wrong in townhome lines

The pipe is modern PVC and the failures are almost all about installation and later damage. Long flat runs under driveways settle into bellies. Utility trenches for fibre or gas cut across the line and are backfilled loosely. Construction debris — mortar, drywall mud, a lost cleanout plug — sits at a low point and catches everything. And in rows built during the 2000s boom, the shared line was sometimes undersized for the units eventually connected. None of these are exotic, and all of them show on camera.

Where this comes up most

Townhome rows are dense in Ballard, Fremont, Capitol Hill, Beacon Hill, Lake City and the West Seattle junction area, and increasingly across Shoreline and Kirkland. If you manage or sit on the board of a townhome HOA, the property-manager article is about keeping the shared-line records that make the next repair a decision instead of an argument.