Planning Board backs ADU bylaw clean‑up, debates 900‑ vs. 1,200‑sq‑ft cap

The Nantucket Planning Board on Jan. 29 recommended most of a multi‑article package that inserts the state’s protected‑use accessory dwelling unit (ADU) definition into town zoning and cleans a slate of related definitions and use‑chart entries.

Town counsel Amy Questle told the board the state statute and implementing regulations treat a “protected‑use ADU” as an ADU that is “900 square feet or half of the principal dwelling, whichever is smaller,” and that local bylaws must be reconciled with that language. Staff proposed a series of edits (articles 47–54) to remove older accessory‑dwelling language, merge gross‑floor‑area definitions, delete the tertiary‑dwelling term, and relocate certain commercial‑district uses into apartment density rules.

Why it matters: The state change grants property owners an as‑of‑right ADU in many single‑family zones, and the town’s edits are designed to preserve Nantucket’s preexisting limits on density and occupancy while avoiding inadvertent increases in the number of kitchens or units on a lot. The package seeks to keep density consistent with prior town meeting votes but also to clarify which structures count for size calculations and how the building commissioner should measure gross floor area.

Board debate centered on two policy choices. Some members urged keeping the state model: a protected ADU capped at 900 square feet, which counsel and staff said clearly reflects the statute and reduces legal risk. Others urged a more permissive local cap — for example, 1,200 square feet — to legalize more existing secondary dwellings and reduce incentives for property owners to seek a third dwelling or additional kitchens by circuitous means. Amy Questle cautioned that adopting an as‑of‑right figure larger than the statute’s 900 could invite challenges from property owners who claim the statute guarantees a 900‑square‑foot ADU under state law.

On technical points, the board clarified that gross‑floor‑area exclusions (crawl spaces, garages, attics, enclosed porches and similar spaces) apply to calculation of the principal dwelling’s size — which in turn determines the maximum allowable ADU size under the ‘50 percent of the principal dwelling’ rule — and that the building commissioner will apply the state measurement standard in permit review.

Votes and next steps: After discussion the board closed public comment and voted positive recommendations on multiple articles as drafted, including Articles 47 (replace the local accessory‑dwelling definition) and 48 (insert the state protected‑use ADU definition). The board also recommended Articles 49, 52, 53 and 54 (the GFA merge, tertiary elimination, removal of the garage‑apartment term, and apartment density cleanups). Some articles that required re‑advertisement or additional notice were continued to the Feb. 9 session for final consideration. Staff said towns will likely revisit fine‑tuning after the state’s new law and guidance settle.

What board members said: Amy Questle summarized the statutory change and the key measurement rule: “protected use ADU is an accessory dwelling unit that is 900 square feet or half of the principal dwelling, whichever is smaller.” Board member Hillary Rayport said she supported clarifying that only one protected ADU can exist per lot and urged staff to consider whether enlarging the ADU size might better capture existing secondaries. Planning director Megan Trudeau said she worked with the building commissioner and town counsel to draft language intended to preserve the town’s prior density outcomes while complying with state law.

The packet of recommended articles will go forward to town meeting with staff and counsel’s explanations; the board asked staff to prepare clear, plain‑language summaries for voters to reduce confusion on the floor.