How to Connect a Sewer Line to Your Accessory Dwelling Unit (ADU)
When adding an Accessory Dwelling Unit (ADU) to your property, connecting the sewer line is one of the biggest challenges. Unlike water and electrical lines, which can be routed almost anywhere, a sewer line requires careful planning to ensure proper drainage. This is because wastewater relies on gravity flow, meaning the pipe must maintain a ¼ inch drop per foot of run to function correctly.
If you don’t have enough natural slope, you may need to install a sewage ejector pump to move wastewater from your ADU to the main sewer line.
This guide will walk you through the key steps to determine how and where to connect your ADU’s sewer line, including what to do if gravity flow isn’t an option.
Step 1: Locate Your Existing Sewer Line
Before you plan anything, you need to find out where your home’s main sewer line runs. There are two common ways to locate it:
1. Find the Sewer Cleanout
The cleanout is an access point to your home’s main sewer line, usually found:
• Near the foundation of your home, often in the front or backyard.
• Alongside the house, in a capped pipe sticking out of the ground.
• Inside the basement (if you have one).
If you can find the cleanout, it gives you a good idea of where your main sewer line runs and where your ADU’s sewer connection might tie in.
2. Use a Plumbing Camera Inspection
If the cleanout isn’t visible or you want precise details about your sewer line’s path, hire a plumber with a video scope. They can insert a camera into the cleanout or another access point to:
• Identify the exact location and depth of the sewer line.
• Map out its direction and where it connects to the city sewer system.
• Check for any blockages or obstructions.
Step 2: Determine Your ADU Sewer Route
Once you know where your main sewer line is, you’ll need to plan the route from the ADU to that line. Here’s what to consider:
1. Ensure a Proper Slope (Rise to Run Ratio)
Your sewer line must maintain a ¼ inch drop per foot of run for gravity to carry wastewater to the main line. To calculate if your ADU site has enough slope:
• Measure the elevation difference between your ADU’s planned sewer exit point and where the main sewer line will connect.
• Measure the horizontal distance between these two points.
• Multiply the total run in feet by ¼ inch to determine the required drop.
For example, if your ADU sewer pipe needs to run 40 feet, you’ll need a minimum vertical drop of:
40 ft x ¼ inch per foot = 10 inches of total drop.
If your property lacks the necessary slope, you’ll need to install a sewage ejector pump, which we’ll discuss in Step 3.
2. Choose the Best Connection Point
There are usually three ways to tie your ADU’s sewer into the main line:
1. Existing Cleanout – If allowed by local code, you might be able to connect directly to the cleanout near your home.
2. Lateral Connection – Cutting into the main sewer lateral line between your home and the street.
3. Main Line Tap – If required, you may need to tap directly into the municipal sewer at the street (this is the most complex and costly option).
Step 3: Install the Sewer Line (Gravity or Pumped System)
Once you’ve mapped out the connection point and slope, you have two installation options:
Option 1: Gravity-Fed Sewer Line (If Slope Allows)
If your ADU is at a higher elevation than your main sewer line, you can use a gravity-fed system:
1. Excavate the trench along the planned sewer route, ensuring you maintain the correct slope.
2. Lay the sewer pipe, typically 4-inch PVC (Schedule 40), following local plumbing codes.
3. Connect to the main sewer line using a proper wye (Y) fitting and a rubber coupling.
4. Secure and test the system before backfilling the trench.
Option 2: Sewage Ejector Pump (If Gravity Isn’t Possible)
If your ADU sits below the elevation of the main sewer line, a sewage ejector pump will be necessary to move wastewater uphill. Here’s how it works:
• Wastewater from the ADU flows into a sealed pump basin (typically located below ground or in a crawlspace).
• Once the basin fills to a certain level, the pump automatically activates to push the wastewater through a pressurized pipe.
• The discharge pipe connects to your home’s existing sewer lateral or a designated tie-in point.
Considerations for a Pumped Sewer System
• Power Requirement – The pump requires an electrical connection, meaning it won’t function during a power outage unless you have a backup system.
• Pump Capacity – Choose a pump that can handle the expected wastewater load from your ADU (typically rated in gallons per minute).
• Check Valves & Venting – The system needs a check valve to prevent backflow and proper venting to allow air into the drainage system.
Final Considerations
• Check local codes – Some municipalities require a backwater valve to prevent sewer backups into your ADU.
• Get permits – Sewer connections usually require approval from your local city or county.
• Hire a professional – If unsure about slope calculations, local codes, or installation, consult a licensed plumber.
Conclusion
Connecting an ADU to a sewer line requires careful planning, especially ensuring the correct rise-to-run slope for gravity drainage. Start by locating your existing sewer line—either by finding the cleanout or using a camera inspection—then determine the best connection point and trench path.
If gravity-fed drainage is possible, maintain a ¼ inch per foot slope and connect to the main sewer line with proper fittings. If gravity flow isn’t possible, install a sewage ejector pump to move wastewater uphill to the main sewer.
Taking these steps early will help you avoid costly mistakes and ensure your ADU’s plumbing functions smoothly for years to come.