ADU Plumbing DFU and Sewer Sizing Guide

Common ADU DFU Examples

ADU fixture loads vary depending on whether the unit has laundry, multiple bathrooms, a dishwasher, or other fixtures. The table below shows simplified examples for early planning. To build a custom fixture list, use the DFU calculator or compare individual fixtures with the DFU chart.

On mobile, scroll the table sideways to view all columns.

Common ADU Fixture DFU Values

Most ADUs include a bathroom and kitchen. Some also include a clothes washer, laundry sink, dishwasher, garbage disposal, or floor drain. Each fixture adds to the total drainage load. For bathroom-specific examples, review the bathroom DFU guide and the toilet DFU guide.

Example: ADU DFU Calculation

Here is a simple ADU example using common UPC-style fixture values. Assume the ADU includes one bathroom, a kitchen, a dishwasher, and a clothes washer.

This example does not include a garbage disposal, laundry sink, floor drain, second bathroom, bathtub, or future fixture additions. If the ADU shares the main house sewer, calculate the combined load from both the house and ADU.

House Plus ADU Sewer Load

The most important ADU sewer question is often not the ADU by itself. It is whether the existing building drain, building sewer, or private lateral can handle the combined load from the main house and ADU.

3-Inch vs 4-Inch Sewer for an ADU

A 3-inch sewer may be adequate for some house-plus-ADU projects, but the answer depends on total fixture load, pipe slope, pipe classification, and local code. A 4-inch sewer may be worth reviewing when the combined load is high, the sewer run is long, the slope is limited, or future expansion is likely.

Pipe condition matters too. An old 4-inch sewer with roots, bellies, offset joints, cracks, or poor slope may perform worse than expected. Diameter alone does not prove usable capacity.

Sewer Slope and ADU Connections

ADUs often create sewer routing challenges. The ADU may be behind the main house, uphill, downhill, detached, converted from a garage, or connected through an existing sewer line. The available slope and connection elevation can determine whether the proposed route works.

Long runs, low-slope sections, shallow sewer mains, and existing pipe depth can all affect the design. Before trenching, confirm the start elevation, connection elevation, run length, cleanout locations, and required slope. To estimate fall over distance, use the sewer slope calculator.

Building Drain vs Building Sewer for ADUs

When reviewing an ADU, identify where the ADU connects to the existing system. It may tie into an interior building drain, an exterior building sewer, a private lateral, or a separate sewer connection. Each option can have different code and inspection implications.

If the ADU connects to the main house plumbing before the pipe exits the structure, the building drain may need review. If the ADU connects outside the building, the building sewer or private lateral may be the focus. For a terminology breakdown, review building drain vs building sewer.

Common ADU Plumbing Review Questions

• How many fixtures will the ADU include?
• Will the ADU have a clothes washer or laundry sink?
• Will the ADU share the main house sewer?
• Where will the ADU tie into the existing drainage system?
• What is the existing sewer size?
• What is the existing sewer slope and condition?
• Are cleanouts available and properly located?
• Does the project fall under UPC, IPC, or local amendments?
• Will the building department or utility require a separate sewer connection?

Common ADU DFU and Sewer Mistakes

• Calculating only the ADU and ignoring the existing house load.
• Assuming a 3-inch sewer is always enough for a shared system.
• Assuming a 4-inch sewer is adequate without checking condition.
• Forgetting laundry fixtures, dishwasher, disposal, or floor drains.
• Ignoring sewer slope, long runs, and connection elevation.
• Missing cleanout access on the ADU connection.
• Using UPC values in an IPC jurisdiction, or IPC values in a UPC jurisdiction.
• Not checking local ADU plumbing, sewer, or utility requirements.

Assumptions and limitations

The ADU examples on this page are simplified planning examples. They are intended to help estimate fixture load and identify sewer sizing questions before permit review.

• Actual ADU fixtures may differ from the examples shown.
• Local code editions and amendments can change the result.
• Public-use or commercial-style fixtures may need separate review.
• Separate sewer connections may be required in some jurisdictions.
• Existing sewer condition can be more important than diameter on paper.
• Final design should be verified with the local authority having jurisdiction.

To check which plumbing code may apply, start with the state plumbing code lookup.

Frequently Asked Questions