Southborough, MA Building Safety: Anti-Mansionization (2026)

Chapter 174 does not use the word mansionization. Its control on house size is the floor area ratio, defined in § 174-2 as the ratio of the aggregate gross floor area of all floors of a building or buildings on a lot to the total lot area. The gross floor area shall not include unenclosed porches, cellars, attics or garages not used for human occupancy.

Section 174-8.2(D)(5) sets the maximum floor area ratio in the Residence A district at 0.18, alongside a minimum lot area of 43,560 square feet under § 174-8.2(D)(1), with at least 20,000 square feet exclusive of wetlands. Section 174-8.3(D)(5) sets Residence B at .30, with a minimum lot area of 25,000 square feet. Lots laid out and recorded in Residence B districts before the 1966 discontinuance of Residence C districts are built under § 174-8.3(E)(5) with a maximum floor area ratio of 0.45 and a minimum lot area of 15,000 square feet.

The companion definition of FLOOR AREA, GROSS measures the aggregate horizontal area of all floors of a building or several buildings on the same lot from the interior faces of the walls enclosing each building, exclusive of stair and elevator wells, garages, basement and other areas used only for storage or for services incidental to the operation and maintenance of the building. Because the ratio counts all buildings on the lot, an accessory building adds to the total.

The ratio works with the other residential standards: setbacks, a 35 foot, 2 1/2 story height cap and a building-lot circle test. For existing houses, § 174-19(B) lets a single- or two-family owner alter or extend as of right when the change complies with all current setbacks, lot coverage and building height requirements, so an addition on a conforming lot is still checked against the standards. Section 174-14 requires that any building hereafter located, erected, commenced, expanded or altered comply with the chapter.