Town of Northborough health officials on March 11 discussed how a recent state law making accessory dwelling units (ADUs) a right in many residential zones affects local public-health review of septic systems and minimum habitation standards.

Chris, a health department staff member, told the Board of Health that "the Affordable Homes Act" (as described in the meeting) supersedes local ADU bylaws in some respects and sets statewide limits: ADUs may be no larger than half the square footage of a primary dwelling or 900 square feet, whichever is smaller. He said the law also curtails some local restrictions and leaves room for what the state calls "reasonable restrictions." Title 5 — the state's septic regulation — was specifically called out as a reasonable restriction that the department will apply in permitting decisions.

Officials said the board is awaiting guidance from the Massachusetts Department of Environmental Protection and related agencies about how Title 5 will apply to ADUs, including whether a new or secondary septic system must meet the standard minimum sizing (Chris said "typically has to be 330 gallons per day with that capacity" for a new system) or whether smaller or cumulative systems can be approved.

Chris told the board that residents on public sewer are not restricted in the same way as those on private septic, but that many Northborough lots are small and could pose practical limits on adding ADUs. The planning and building departments are preparing proposed local regulations to present at the town's annual meeting to codify "reasonable restrictions" under the new state law.

The board agreed to monitor guidance from state agencies and to coordinate with planning and building officials before any local permitting or policy changes are finalized.