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Planning board reviews state ADU rule changes; town staff identify measurement and definition issues
Summary
Concord planning staff told the Planning Board on Jan. 14 that the town’s draft ADU bylaw mostly complies with new state ADU regulations but that technical differences in how gross floor area is measured and whether attics/porches count must be resolved.
Town planning staff told the Planning Board on Jan. 14 that Concord’s draft accessory dwelling unit (ADU) bylaw generally aligns with newly issued state regulations but that a few technical differences warrant coordination with the Building Commissioner and Town Counsel.
Staff reviewed the state definition for gross floor area used for ADU eligibility and noted two points: Massachusetts’ recent regulation measures gross floor area from the interior face of exterior walls (and excludes garages, crawl spaces, attics, enclosed porches and “similar spaces”), while Concord’s bylaw measures from the exterior face of exterior walls. That difference can change the measured square footage by roughly a few dozen square feet depending on wall thickness, staff said. The board discussed whether Concord should adjust its definition or otherwise ensure the ADU threshold (the draft regulation sets a 900 sq. ft. baseline) will match practical outcomes given measurement differences.
Planning staff also reported that the state clarified how multiple ADUs on a single lot are handled: a municipality that allows additional ADUs on the same lot would require a special permit for those extra ADUs under the state rule. Town Counsel advised that Concord’s current ADU bylaw does not permit multiple ADUs on a single lot, so that particular state provision need not change Concord’s text now.
Several residents asked why the draft includes a 1,000 sq. ft. maximum when the state baseline is 900 sq. ft. Staff said the board previously set the larger figure to avoid requiring a special permit for units that marginally exceeded the lower threshold due to measurement conventions and historical precedents; staff recommended keeping the higher figure to reduce the number of special‑permit reviews for small overage cases. Staff also said they will consult with the Building Commissioner about whether attics or enclosed porches that have been converted to conditioned, habitable space must count toward ADU gross floor area under the state rule.
The board did not vote on ADU language at the meeting and will continue to coordinate with Town Counsel and the Building Commissioner before the Feb. 11 meeting.
