The Chilmark planning board has the authority to regulate large houses through a special permitting process, but cannot simply place a cap on house size, the town attorney advised this week.

At the planning board meeting on Monday, town counsel Ronald H. Rappaport presented a legal framework for the planning board as it works to develop possible zoning regulations governing very large houses in town.

“The issues of house sizes and how far you can limit them is obviously an area which communities around the state are dealing with,” Mr. Rappaport said. “I feel it is my job to call it as I see it and tell you how far I think you can go.”

He said establishing a square footage threshold for large houses that triggers extra review from town land use boards is allowable, but an outright cap on house size is not.

Mr. Rappaport discussed the advantages of adding zoning regulations under existing town districts of critical planning concern, special overlay planning districts created through the enabling legislation of the Martha’s Vineyard Commission. He said such zoning rules have extra legal strength due to the fact that they were created through the MVC, a regional planning agency with powers that go beyond those of local land use boards. He said under a DCPC, the town could choose to create a planning board plan review committee, like the one in Aquinnah. Aquinnah voters adopted a townwide DCPC in 1999; in 2007 the town added rules to govern the size of houses.

A Chilmark planning board subcommittee has been meeting regularly since last January to develop a bylaw that regulates large houses. The current draft, originally envisioned as a two-year temporary bylaw, now calls for a permanent bylaw requiring houses over 3,500 square feet to go through an extra layer of review.

Modifying existing DCPCs could be the way to go, Mr. Rappaport said. DCPCs cover a “significant segment of the most prominent and the most fragile and the most important areas of town,” the town counsel said. “Voters have said so when they adopted the DCPCs and the MVC has said so because they approved them.”

In Chilmark there are town-specific DCPCs for Meetinghouse Road and the Tiasquam River, Menemsha, Nashaquitsa, Squibnocket and Stonewall Ponds, Waskosim’s Rock and the north shore. There are also Islandwide DCPCs that include Chilmark, covering roads and coastal areas.

Mr. Rappaport encouraged the board to develop regulations “that are not only effective but have the best chance of surviving a legal challenge.”

“A lot of deference has been given [by the courts] to the DCPCs created by the MVC,” Mr. Rappaport said. “If you were to consider moving these regulations into DCPCs themselves, you would get the enhanced benefit of the commission’s legal authority.”

He continued: “The thought that we might adopt a bylaw which would be free from legal scrutiny or court challenge would be naïve to assume. I’m trying to make a practical suggestion that could enhance the likelihood that any court challenge would be upheld.”

Zoning regulations governing house size have attracted lawsuits from home builder associations and realtor groups, Mr. Rappaport said.

“We don’t shy away from fights, but on the other hand you want to protect yourself in terms of possible legal challenges because you’re not engaged in a pure planning exercise, you want these to work and you want them to be upheld,” he said.

He also said the board is well within its rights to develop triggers for special permits by using a floor area ratio [FAR] to determine square footage based on lot size, “provided your limits to begin with are reasonable.” He said his research into the issue included consulting with town counsels from Wellfleet, Barnstable and Brookline, which have all recently adopted some kind of large house regulations. He also spoke to land use planners at the Cape Cod Commission.

The state Supreme Judicial Court upheld the Brookline zoning board of appeals in a challenge to its bylaw that uses floor area ratio to regulate single-family homes.

The case (81 Spooner Road vs. Brookline Zoning Board of Appeals) was upheld by the court because the effect of the mathematical equation dictating house size was incidental and not direct, Mr. Rappaport said. Restricting the interior dimensions of a house is not legal, he said.

“I do believe you can create a cap with a special permit review to follow, and as long as your criteria for the special permit are objective enough, and that’s a big loophole, that a cap with a special permit review would be upheld by court under the same reason as 81 Spooner Road,” Mr. Rappaport said. “An absolute cap would not be.”

Mr. Rappaport said varying lot sizes in town and across the Vineyard can make it difficult to apply the floor area ratio rule.

“That’s one of the reasons why the FAR are really not helpful on the Vineyard,” he said. “It may be helpful on certain lots but they just don’t help much on large lots.”

Selectman Warren Doty called Mr. Rappaport’s review “an intelligent approach.”

“I think the approach you’ve taken, which is new to me today, is that you’re not just making regulations that the attorney general will approve . . . but a bylaw that will stand up to a challenge,” Mr. Doty said. “This board and this town have done a very good job of controlling subdivisions — that’s not our problem at the moment. The thing you’re seeking to control, or thinking about, is house size.”

Board chairman Janet Weidner said later that the planning board hopes to have a bylaw ready for the annual town meeting in April. Ms. Weidner said the subcommittee may decide to use existing DCPC regulations and some sort of formula to regulate house size.

“We may end up with a little bit of both,” she said.

“It was a good opinion [from Mr. Rappaport] and we’d be foolish not to go with it,” she added.