AMHERST — Large stretches of undeveloped land set aside decades ago for research parks could instead become apartment buildings and townhouses under a zoning proposal now being explored by the Planning Board.

Observing that he has met with developers and the board’s housing subcommittee, Planning Board member Angus McLeod said the concept is to rezone the so-called PRP zones to spur the creation of mixed-use buildings and medium density housing.

“They have not seen the kind of development that they were designed to foster, to the degree we had hoped for, when they were created in previous decades,” McLeod said at the board’s July 1 meeting.

The zoning change pitched has three elements. The most complicated is to allow all professional research park zones, first created by Town Meeting in 1980 to accommodate high technology and light industry, to have office park uses, and to permit by-right construction of townhouses and apartments. The change would have no cap on the number of units that can be in a building and in mixed-use developments.

The other elements are to change the office park designation to limited business and to eliminate a separate office park district. This would affect a portion of University Drive where medical offices are located, but McLeod said that area is already included in an overlay district that encourages more in-fill housing development close to the University of Massachusetts campus.

McLeod noted that the PRP zones are clearly not good for housing subdivisions, so concentrated housing seems like the best possible use for them, considering there may be significant wetlands on some of the properties.

Walker Powell, a town planner, said a professional research park parcel on Greenleaves Drive, off Route 9 near the Hadley town line, may be the most ideal spot for such a zoning change. Greenleaves Drive already has five apartment buildings with 39 units each, so raising the cap on dwelling units, or eliminating that cap entirely, would make sense for vacant land in that area.

That senior housing development came about in the 1990s with approval from the Planning Board, whose members at the time suggested identifying additional professional research park land elsewhere in town.

It is uncertain whether the professional research park land between Sunderland Road and Montague Road would become housing, with some of that already used for large-scale solar arrays and recently being eyed for a senior and affordable housing by Beacon Communities before those plans were abandoned. And the same zoned land off Belchertown Road has been developed with office buildings constructed on Research Drive and where the Amherst Dog Park is built on top of a portion of the former landfill.

McCleod said it’s great to allow current uses on these sites, but he sees the importance of new residential uses that will meet the demand for housing that exists in Amherst.

Board member Bruce Coldham called McLeod’s idea a “sold conceptual proposal for a change” and that the proposal is ready for other entities to examine, such as the Community Resources Committee of the Town Council, and for Building Commissioner Rob Morra to offer input.

Powell suggested more work to be done in the next six weeks, such as assessing resource areas and wetlands and reviewing the history for professional research parks, and whether such a zoning change might undermine the purpose of setting aside these properties for tax-based development.

Any zoning change would have to go through a process, including advertised public hearings and a favorable vote by the Town Council.