Side sewer basics for backyard cottages.

Ask anyone who has built a backyard cottage what surprised them, and the side sewer comes up more often than the kitchen. It is out of sight, it is the owner’s responsibility, and its route across the lot can move the price by tens of thousands of dollars. Here are the basics to settle before design.

What a side sewer is, and who owns it

The side sewer is the private pipe that carries wastewater from your buildings to the public sewer main in the street or alley. It belongs to the property owner, who is responsible for maintaining and repairing it all the way to the main. When you add an ADU, the new home needs a path into that system.

Tie in or run new

Before you assume you can tie in, have the existing side sewer scoped with a camera. An old clay or cracked line under the house is better found now than after the cottage is built.

Gravity or pump

Sewers work best by gravity: a continuous downhill slope from every fixture to the main. If the cottage sits lower than the main, or the line would have to climb over an obstacle, a pump system is needed. Pumps work, but they add equipment, power and maintenance. On many lots the best location for the cottage is simply the one that drains by gravity.

Why the route matters

• Length. Every foot is excavation, pipe, bedding and restoration.
• Obstacles. Trees and their roots, driveways, retaining walls and the house’s own foundation.
• Access. Narrow side yards mean smaller equipment and slower digging.
• Neighbors. If the only workable route crosses someone else’s land, you need a recorded easement.
• Shared trenches. Water and power can often share the trench, which is why planning all three together saves money (Utilities and connections).

Permits in Seattle: SPU since October 2025

On October 1, 2025, side sewer work in Seattle moved from SDCI to Seattle Public Utilities. SPU’s Development Services Office now reviews side sewer plans and issues permits, and SPU inspectors inspect the work. Applications still go through the Seattle Services Portal. Core tap fees now appear on the side sewer permit invoice and are paid up front. Onsite drainage review stays with SDCI as part of the building permit. Questions: [email protected] or (206) 684-5362.

The capacity charge: a cost after the build

In King County, a property that adds a new connection to the regional sewer system pays a capacity charge, billed monthly for 15 years. For connections from January 1, 2026, it is $77.99 per residential customer equivalent per month, and an ADU counts as 0.59 RCE: about $46 a month. It can be paid off early at a discount.

Five questions to answer before design

1. Where is the sewer main: street or alley, and how deep?
2. What condition is the existing side sewer in (camera scope)?
3. Can the cottage drain by gravity from where you want it?
4. What is in the way along the route?
5. Will you ever sell the ADU separately? If so, plan separate services now.