ADU Plumbing Guide: Planning & Installing Pipes for Backyard Cottages

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So, you’re thinking about adding an ADU or a backyard cottage. Smart move. It’s one of the best ways to add space, rental income, or a home for family. But let’s be honest—the glamour is in the finishes. The real make-or-break work? It’s often hidden behind the walls, specifically in the plumbing.

Getting the pipes right is like building a solid foundation for your circulatory system. Mess it up, and you’ll have headaches for years. Do it well, and everything just… flows. This guide walks you through the planning and installation maze, from the first sketch to the final turn of the wrench.

Table of Contents

The Blueprint Phase: It’s More Than Just Lines on Paper

Jumping straight to digging is a costly mistake. The planning stage is where you save thousands and avoid future nightmares. You need to answer some big questions first.

Key Questions to Answer Before Any Pipe is Laid

Connection Point: Will you tie into the main house’s existing plumbing, or run a completely separate line from the municipal sewer and water main? The first is often cheaper, the second offers more independence.
Fixture Layout: Where will the kitchen, bathroom, and laundry (if any) go? Grouping these “wet walls” back-to-back or stacked vertically saves a ton on materials and labor.
Local Codes & Permits: This is non-negotiable. ADU plumbing codes can be surprisingly specific about pipe sizes, venting, and shut-off valve locations. Your local building department holds the map here.
Soil and Slope: The lay of your land dictates your drainage. A gravity-fed sewer line needs a consistent downward slope—usually 1/4 inch per foot. No slope? You might need a grinder pump system, which is a whole other ballgame.

The Installation Path: Tying In vs. Going Solo

Here’s the deal: your connection strategy shapes your entire project’s budget and complexity. Let’s break down the two main paths.

1. Tying into the Main House System

This is common for detached ADUs that are relatively close to the primary residence. You’re essentially extending the home’s existing circulatory system. You’ll tap into the main water supply line and the main sewer or septic line.

Pros: It’s usually less expensive upfront because you’re not running new lines all the way to the street. You’re also working with known connection points.

Cons: You add load to your home’s existing system. Is your water heater sized for two households? What about your sewer line’s capacity? Older homes might need upgrades to handle the extra demand. And, well, if you need to shut off water to the ADU, you might be shutting it off to the main house too unless you install separate shut-offs.

2. Installing a Separate Service Line

This means running brand new water and sewer lines from the city connections at the street directly to your backyard cottage. It’s like the ADU is its own tiny house, utilities-wise.

Pros: Total independence. It doesn’t strain the main house systems, allows for separate metering (big for renting), and makes future repairs or modifications simpler. It often adds more value, too.

Cons: It’s more expensive. You’re paying for more trenching, more pipe, and separate tap fees to the city. You’ll also need space on your property for the new lines, which might mean tearing up that nice driveway or garden.

Material Matters: Picking Your Pipes

Copper, PEX, PVC, ABS… it can sound like alphabet soup. Here’s a quick, real-world rundown.

Material	Best For	Human-Friendly Notes
PEX	Water supply lines	Flexible, easy to install, resistant to freezing bursts. It’s the go-to for most modern ADU projects. Comes in red (hot) and blue (cold).
PVC	Drainage, waste, vent (DWV)	Lightweight, inexpensive, and easy to cut & glue. Used for everything except carrying pressurized hot water. It’s the white pipe.
Copper	Water supply lines	Durable and time-tested, but pricey and requires soldering skills. Still loved for its reliability, but PEX is often the more economical choice.
ABS	Drainage, waste, vent (DWV)	Similar to PVC but black. Check your local code—some regions prefer one over the other. It’s a bit more flexible.

The Hidden Hero: Proper Venting

If drainpipes are the intestines of your ADU, the venting system is the lungs. Honestly, it’s the part most DIYers misunderstand. Every drain needs a vent to prevent sewer gases from coming into the home and to allow wastewater to flow smoothly.

No vent, or a bad vent, and you’ll get slow drains, gurgling sounds, and maybe even siphoned water from P-traps (that’s the smelly part). For a simple bathroom group in an ADU, an Air Admittance Valve (AAV) can sometimes be a lifesaver—it’s a one-way mechanical vent that allows air in when needed, saving you from running a physical vent pipe through the roof. But—and this is a big but—not all local codes allow them, so always check.

Real-World Challenges & Smart Solutions

Let’s talk about the gritty stuff, the things that pop up when you’re actually in the trench.

The Frost Line: Water supply and sewer lines must be buried below the frost line in your area. That depth varies wildly. In Minnesota, you’re digging deep. In Southern California, not so much. This dictates your trenching cost and effort.
Insulation is Not Optional: Especially for ADUs on piers or with exposed plumbing. Insulating hot water lines saves energy. Insulating all lines in crawl spaces prevents freezing. Use foam pipe insulation—it’s cheap insurance.
Access Panels are Your Friend: That sleek wall hiding the shower valve? Make sure there’s an access panel behind it. Trust me, the next person who needs to fix a leak will thank you. And that person might be you.
Water Heater Strategy: A tankless water heater mounted on an exterior wall can save precious interior square footage in a small backyard cottage. Or, a small point-of-use heater under the kitchen sink can deliver instant hot water without running a long line from a central tank.

A Final Thought: It’s an Investment in Invisible Comfort

When your ADU is finished, no guest will marvel at the perfect 1/4-inch slope of your drain line. They’ll notice the beautiful tile, the cozy light. And that’s the point. Good plumbing is silent, reliable, and utterly invisible. It’s the quiet promise that when someone turns the knob, warm water arrives. That when they flush, things simply disappear.

Planning it right—mapping it, permitting it, choosing materials for the long haul—isn’t just about avoiding disasters. It’s about building something that endures, that provides comfort without ever asking for applause. That’s the real foundation of a home, whether it’s 200 square feet or 2,000.