Southborough, MA Accessory Structures: ADU Rules (2026)

Chapter 174 defines an accessory dwelling unit as a self-contained housing unit with sleeping, cooking, and sanitary facilities on the same lot as a principal dwelling. The definition requires a separate entrance, either directly from the outside or through an entry hall or corridor shared with the principal dwelling, sufficient to meet the state Building Code for safe egress. It also states that accessory dwelling units shall not be located in a travel trailer.

Section 174-8.2(A)(11) lists, among the permitted uses in the Residence A district, an accessory dwelling unit that is not larger in gross floor area than one half the gross floor area of the principal dwelling or 900 square feet, whichever is smaller. Section 174-8.3(A) makes the permitted uses of the Residence B district the same as Residence A, so the same by-right allowance reaches Residence B lots. Section 174-8.2(B)(1) moves any accessory dwelling unit whose gross floor area is greater than 900 square feet onto the special permit list.

The special permit track in § 174-9(B) covers accessory dwelling units exceeding the state law dimensional standards in MGL c. 40A, § 1A. The application is referred to two bodies. The Board of Health must certify that adequate provisions have been made, under its requirements, for drainage and for the disposal of sewage and waste from the apartment. The Planning Board reports on the lot, the neighborhood, the effect of the apartment, the adequacy of ingress and egress, any restrictions it recommends and the provisions for off-street parking. The permit issues after both reports are received and considered, or after 35 days elapse without a report. If the Board of Appeals decides differently from the Planning Board's recommendations, its decision must state the reasons.

Three conditions attach. The habitable floor area of the accessory unit shall not exceed 25% of the habitable floor area of the entire dwelling plus that of any accessory building used for the accessory dwelling. No other accessory dwelling unit can exist on the lot. And not more than the required minimum exterior alterations can be made to the one-family house and any accessory buildings, with the site plan and floor plans filed with the Building Inspector before the application goes to the Board of Appeals.