Let’s be honest: when you dream of building an ADU, you’re picturing the finished space. The cozy living area, the sleek kitchen, the peaceful bathroom. You’re probably not daydreaming about sewer line taps and water pressure calculations. But here’s the deal—the plumbing is the hidden circulatory system of your unit. Get it right, and everything flows smoothly. Get it wrong, and you’re in for a world of costly, messy headaches.
Planning and installing plumbing for an ADU isn’t just a copy-paste of your main house’s system. It’s a unique puzzle with pieces like local codes, connection types, and space constraints. But don’t sweat it. We’re going to walk through the process, step-by-step, so you can approach your project—or your conversation with a contractor—with confidence.
The Blueprint Phase: What You Must Figure Out First
Jumping straight to pipe sizes is tempting. Resist it. A little planning upfront saves a ton of rework later. This phase is all about asking the right questions.
Connection Strategy: The Big Decision
How will your ADU connect to water and sewer? Honestly, this is your first and most critical choice. It dictates cost, complexity, and even feasibility.
| Connection Type | How It Works | Pros & Cons |
| Tapped/Extended from Main House | ADU lines branch off the existing home’s main supply and drain lines. | Pro: Often more affordable. Con: Can strain an older main house system. Requires careful capacity checks. |
| Separate, Direct Municipal Connections | ADU gets its own water service line and sewer lateral from the street. | Pro: Independent metering, no impact on main house. Con: Higher upfront cost, more trenching, and permit hurdles. |
| Submetering | Uses the main house’s physical connection but with a separate meter to track ADU usage. | Pro: Fair billing for tenants. Con: Still shares the main supply capacity. |
Decoding Local Codes and Permits
You can’t outsmart the code book. Local building and plumbing codes are non-negotiable. They govern everything—pipe material, venting requirements, fixture counts, and especially backflow prevention. A backflow preventer stops contaminated water from siphoning back into the public supply (or your main house’s water). It’s a big deal for inspectors.
Permitting can be a slog, but it’s your friend. It forces a plan review to catch errors. A tip? Talk to your local building department early. A 15-minute conversation can clarify connection policies and save you weeks of redesign.
The Installation Deep Dive: Lines, Drains, and Vents
Okay, plans are set. Permits are in hand. Now we get physical. Think of the plumbing system in three distinct, yet interconnected, layers.
1. The Supply Lines: Bringing Fresh Water In
These are the pressurized cold and hot water lines. Key considerations here are material and size. PEX (cross-linked polyethylene) tubing is the modern favorite—it’s flexible, resistant to scale, and easier to install with fewer fittings than rigid copper. Sizing matters more than you think. A 3/4-inch main supply line branching to 1/2-inch lines to fixtures is standard, but a long run from the main house might need a larger diameter to maintain pressure.
And about that hot water: will the ADU have its own tankless or tank heater? Or will it tie into the main house’s system? A dedicated, on-demand tankless heater in the ADU is a fantastic trend—it saves space and energy, and gives the tenant full control.
2. The Drain-Waste-Vent (DWV) System: Getting the “Used” Water Out
This is the true engineering marvel. DWV pipes rely on gravity, not pressure. They’re larger, sloped precisely (usually 1/4 inch per foot), and, crucially, vented.
Let’s talk vents. Every drain needs a vent pipe that pokes through the roof. Why? It prevents sewer gases from entering the living space and, more importantly, it allows wastewater to flow smoothly without creating a vacuum that siphons water from P-traps (those U-shaped bends under every sink). If you hear a gurgling drain, that’s a venting problem.
3. The Fixture Install: Where It All Comes Together
This is the satisfying part—hooking up the sinks, toilet, shower, and washer. Layout efficiency is king here. Try to group the kitchen, bathroom, and laundry close together, or better yet, stack them vertically if it’s a two-story ADU. This “wet wall” strategy consolidates plumbing into one chase, saving materials, labor, and future repair headaches.
Choose fixtures wisely. Low-flow toilets and WaterSense-labeled faucets aren’t just eco-friendly; they reduce demand on your entire system, which is a smart move for a shared water supply.
Common Pitfalls and How to Sidestep Them
Even with a great plan, things can… go sideways. Here are a few real-world snags.
- Underestimating the Main House’s Capacity: An older 3/4-inch main water line might struggle to supply both homes at peak use (think: morning showers). A pressure test and load calculation are cheap insurance.
- Ignoring Future Access: Burying pipes in a wall without an access panel is asking for trouble. Plan for cleanouts and panels, especially for key shutoff valves.
- Going Too DIY on the DWV: Supply lines are one thing. The drain-waste-vent system? That’s where professional install pays for itself. A single bad slope or missing vent can haunt you.
- Forgetting About Frost Depth: If you’re in a cold climate, any exterior water line—even just the run from house to ADU—must be buried below the frost line. Period.
Final Thoughts: More Than Just Pipes
In the end, plumbing an ADU is a lesson in foresight. It’s about building resilience and independence into a small space. That careful planning you do for the pipes? It echoes in the quality of life for whoever calls the unit home. A strong, silent flow when they turn the tap. A quick, efficient drain. No surprises.
It transforms the ADU from a dependent appendage into a truly accessory dwelling—a complete, self-sufficient home that just happens to share a piece of land. And that, you know, is the whole point.
