Montgomery County's agricultural preservation plan is moving closer to reality, with the first sales of farmland development rights expected to come within the next few months, according to county planning officials.

Farmers apparently already are signing contracts with builders to sell the transferable development rights, called TDRs, which builders will be able to exchange for land in "receiving areas" soon to be designated by the County Council in eastern parts of the county, according to Dale Price, chief of research for the Montgomery County Planning Board.

Designed to limit development and help preserve 73,000 acres of rural up-county farmland, the TDRs can be used by developers to build houses in greater numbers than otherwise allowed by the zoning laws in parts of the county where the County Council decides there are adequate public services such as schools, sewers and roads to handle additional development.

Up-county farmers acquired about 10,000 TDRs last fall when their land was zoned to allow construction of one house on 25 acres instead of the five houses permitted under the previous zoning. For the four additional houses that could have been built, the farmer gets four TDRs which he can sell. While some county officials have estimated the value of a TDR at $6,000 to $12,000, their market value is still unknown since no sales have taken place. Those farmers who have contracts to sell their TDRs are keeping the numbers "close to their chest," Price said.

The County Council next month is expected to permit use of close to 3,500 TDRs in eastern parts of the county -- or 3,500 additional single-family houses -- when it approves official "receiving areas" there. County planners are now studying additional receiving areas around Damascus, to allow 400 to 500 more houses there than permitted under present zoning. Gaithersburg and Potomac and half a dozen other sectional master plans also may be amended in the next year to include receiving areas and slight increases in housing density, according to John Hoover, spokesman for the Montgomery planning board.

A Development Rights Acquisition Fund established by the county will be used to guarantee bank loans to developers who buy TDRs and build houses with them, and also to enable the county to buy TDRs from farmers, when deemed advisable by the fund's board of directors. The county will hold the TDRs until they can be resold to developers. The new board is authorized by the council, but its members have yet to be chosen by the county executive.