The Montgomery County Planning Board Thursday approved high-density zoning that would allow the construction of hundreds of townhouses at Timberlawn, a south Rockville subdivision orignally billed as an enclave of expensive, detached single-family homes.

The board approved development plans for more than 900 attached houses and smaller single-family homes, but required the developer, U.S. Home, to remove some of the planned units, improve landscaping designs and meet other stipulations aimed at creating a buffer between the existing neighborhood of large houses and the rows of high-density buildings planned to the south.

In late negotiations, lawyers for U.S. Home agreed to changes requested by Robert Hillerson, an attorney representing 33 families who already bought homes at Timberlawn valued above $200,000.

Many of those buyers were angered when U.S. Home changed the development scheme from an exclusive neighborhood of 294 large, single-family houses to smaller detached homes and townhouses that would bring in more than 900 families. Hillerson said the Timberlawn Homeowners Civic Association had agreed not to oppose a compromise plan he said he negotiated earlier with U.S. Home attorney Jerome E. Korpeck.

The planning board and its staff told the builders to eliminate some town houses from the plan and move others that were to be built near existing homes, move planned tennis courts and recreational facilities further west and away from the present neighborhood, according to Joseph Davis, a planning and zoning analyst for the board.

The development changes are expected to be complete before Jan. 15, when a county hearing examiner reviews the design. The examiner's recommendation goes to the County Council for final approval later in the process, Davis said.