Building an accessory dwelling unit is one of the smartest investments a Portland homeowner can make right now, but the plumbing side of the project trips up more people than almost anything else. It involves permits, utility connections, and code compliance, not just running a few extra pipes. Getting any of those wrong after framing is underway is far more expensive than planning for them upfront.

Whether you're converting a garage, finishing a basement, or adding a detached cottage in the backyard, the plumbing questions come early and they shape the rest of the project. Here's what to know before you break ground.

What Makes ADU Plumbing Different From a Standard Remodel

An ADU is a fully independent living space, which means its own kitchen, its own water connections, and often its own sewer tie-in. Portland's Bureau of Development Services and the Oregon Plumbing Specialty Code both have specific requirements for how that work gets done.

Most standard remodels move fixtures around within an existing system. An ADU typically means adding a second point of connection to the main water line, routing drain lines to the city sewer or a septic system, and confirming existing water pressure can support the added load.

Portland's ADU permitting page notes that ADU connections often require a separate water meter or an amended service connection, coordinated through Portland Water Bureau by a licensed contractor. Getting this wrong early creates expensive rework down the road.

Permits, Inspections, and the Oregon Plumbing Code

Every ADU plumbing project in Portland requires a plumbing permit. This is how the city verifies a licensed plumber did the work and that it meets code before anyone lives there.

Every plumber working on the project needs a current Oregon Plumbing License, which you can verify through the Oregon Building Codes Division's license lookup. Hiring unlicensed help to save money on an ADU is one of the fastest ways to run into trouble at resale or rental time.

Inspections happen at multiple stages — rough-in before walls close, and final once everything is hooked up. A plumber experienced with Portland ADU work knows what inspectors look for and how to schedule around it.

Common ADU Plumbing Challenges in Older Portland Homes

Craftsman bungalows, 1950s ranches, and mid-century split-levels weren't designed with a second dwelling in mind. That creates a few specific challenges:

• Water pressure. Adding a full bathroom and kitchen to an existing supply line can drop pressure in both units. A plumber assesses the current service line size and, in some cases, recommends an upgrade. See why water pressure drops and what causes it if this already affects your main home.
• Drain line slope and distance. Drain systems rely on gravity, so every foot of horizontal pipe needs a slight downward pitch toward the main sewer line. Getting that slope right across a long basement or backyard cottage run requires careful planning; an incorrect grade means slow drains or backups.
• Water heater sizing. A single tank water heater serving two units is usually undersized. Most ADU projects need a dedicated unit, and many Portland homeowners choose a tankless water heater in the ADU for the space savings and efficiency. Our tankless vs. tank comparison covers the trade-offs.

Basement and Garage Conversions: A Note on Ejector Pumps

Converting a below-grade space into an ADU introduces a challenge that surprises a lot of homeowners: gravity doesn't work in your favor for the drain lines. When a bathroom or kitchen sits below the main sewer line, waste can't flow downhill on its own.

An ejector pump — a sealed system that collects waste in a pit and pumps it up to the main line — solves this. It's standard for below-grade ADU plumbing, and a sump pump and ejector pump installation done correctly is quiet and low-maintenance. A basement ADU can have a bathroom; it just needs the right equipment.

Planning Your ADU Plumbing Before Work Starts

• Know what you're connecting to. Confirm whether the property is on city sewer or septic. Portland properties inside city limits are almost always on city sewer, but outlying areas of Lake Oswego, West Linn, and unincorporated Clackamas County may be on septic, which requires a separate capacity evaluation for an ADU.
• Loop in the plumber before the layout is finalized. Kitchen and bathroom placement in the ADU determines where drain lines, vents, and supply lines run. Changing that after framing starts costs real money; a plumber who reviews the plan early can flag problems before they become expensive.
• Budget for the unexpected. Older homes sometimes have galvanized steel supply lines or clay sewer pipes near the end of their life. When a plumber opens walls or the crawlspace for an ADU project, those issues surface. It’s better to know upfront than to repair or replace aging pipes after a tenant moves in.
• Ask about backflow protection. Portland Water Bureau requires backflow device installation in certain connection scenarios. This is a code requirement that protects the city water supply.

What PDX Plumber Does on ADU Projects

We handle the full scope of ADU plumbing from rough-in through finish: working with your general contractor on layout early, pulling permits, scheduling inspections, and testing the system before we leave. Nolan brings 25+ years of plumbing experience across Portland, Lake Oswego, Tigard, Beaverton, and the broader metro area. Pricing is upfront, and we back our work with a one-year warranty on select projects, plus up to 25 years on qualifying manufacturer warranties.

New customers, veterans, and seniors get 5% off, and financing is available for larger projects. Ready to get your ADU plumbing done right? Call 503-351-6906, reach out at pdxplumber.com, or read what our customers say on Google first.