Updated November 1, 2026. General information only, not legal, engineering, or insurance advice.
An ADU is part of the same facility
MassDEP's guidance treats an ADU as a dwelling that is part of the same facility as the main house for Title 5 purposes. If adding it increases the system's design flow, it is considered new construction and the system must fully comply with Title 5.
Bedrooms drive design flow
Title 5 design flow for homes is based on bedrooms, at 110 gallons per day per bedroom. A one-bedroom ADU added to a three-bedroom house means designing for four bedrooms.
What to do first
- Find your septic records and most recent Title 5 inspection.
- Ask your designer to check the system's permitted design flow before drawing plans.
- Talk to your local Board of Health early, since it reviews septic work.
Get a free quote
If you would like a local pro to take a look, use the form on this page. It is free to ask and there is no obligation. Related: Detached ADU (Backyard Cottage), ADU Design & Build, Garage Conversion ADU.