A proposal presented to the St. Mary's County commissioners this week could reduce by nearly half the number of homes built in rural parts of the county in coming years.

A 10-member committee organized by the county's Chamber of Commerce and including representatives from the building and farming industries proposed a major revision to the county's transferable development rights, or TDR, program.

The current policy, adopted in 2002, allows developers to build with higher densities if they pay to preserve land in other areas. The revisions strive for a similar goal but are intended to preserve more land and ease the process of transferring development rights.

The point of the proposed change, said Joe Densford, chairman of the chamber's legislative affairs committee, is to discourage "rampant runaway growth" in the Rural Preservation District and direct it toward the development districts around Leonardtown and Lexington Park.

The Rural Preservation District accounts for about 178,000 acres, or nearly 80 percent of county land. Because many people who move to St. Mary's want to live in a tranquil rural setting, the land is some of the most popular for development. About 19,100 homes sit on about 71,000 acres of the district, according to county figures.

Planners estimate that at the current zoning of one house per five acres, about 21,400 homes could still be built in the rural district. But the proposed revisions would slash that number to about 10,700 homes, according to documents explaining the proposal.

The county commissioners said they are generally receptive to the recommendation, but some expressed concerns that the change would make it more difficult to find affordable housing in the county.

"Make no mistake about it . . . what we are talking about is increasing the cost of developing" in the Rural Preservation District, said Commissioner Daniel H. Raley (D-Great Mills). "But I would submit that is the only way we are going to curtail some of the growth."

Under current zoning, a developer could build 20 homes on a 100-acre parcel without using TDRs. The proposal would require developers to buy one TDR, which now sell for about $8,000 to $10,000 each, for every lot after the first one -- or 19 TDRs to build the 20 homes. These 19 TDRs would preserve 145 acres elsewhere in the county. Under the current rules no land would have to be preserved.

"If you're going to build in the [Rural Preservation District] you're also going to help preserve land" in the district, said Lisa Ledman of the St. Mary's County Department of Economic and Community Development.

The use of TDRs is attractive to developers because it allows them to build more homes on the same parcel. On that same 100 acres, a developer who bought more TDRs could build up to 33 homes instead of 20.

But because the proposed revisions would increase the cost of development and decrease the number of homes in the rural areas, some likened the changes to a down-zoning and raised concerns that working-class residents may be pushed out of the county.

Commissioners President Thomas F. McKay (R-At Large) said affordable housing was a "major concern." He asked, "How do the workforce folks afford this?"

St. Mary's County has preserved roughly 2,500 acres under the TDR program, which began in 1990 and was revised in 2002, said Donna Sasscer, head of agriculture and seafood development in the county economic development agency.

On Tuesday, for example, the developers of the Leonardtown Farms subdivision, to be located along Route 5 south of the county fairgrounds, purchased development rights from two property owners to increase the allowable density on their 226-acre parcel. The project, by Calvert LLC, was initially allowed 45 homes, but by buying 52 TDRs, the builders can add 26 homes. The transaction preserved 114 acres in the Medley Neck area and additional acreage in the Scotland Beach subdivision, according to county documents.