North Bethesda's largest remaining tract of undeveloped land, located across from White Flint Mall and surrounding the Timberlawn mansion, is being carved up for a 465-unit subdivision of luxury houses that are expected to go on sale this fall for $150,000 to $500,000.

And on a separate section of what was once the 212-acre Corby estate, purchased last year for $9.2 million, another feature of the Timberlawn development is planned - a 101-unit, moderate-rent apartment building with subsidized rents for the elderly.

The $4.2 million project awaits funding approval from the Department of Housing and Urban Development.

This marriage of inexpensive housing for the elderly and large-lot, expensive homes for the wealthy was performed under provisions of the Montgomery County master plan, which requires builders to reserve 15 percent of all housing in developments of 50 units or more for low- or medium-priced housing.

The builder of the high-rise, and the principal developer of the Timberlawn tract, Berger/Berman Builders Inc., of Rockville, was required by the county to build 80 subsidized housing units. The county had hoped for moderate-priced houses, which are in short supply in the county.

Instead, Berger/Berman will build 101 units in a six-story building on the southeast section of the property, which is included in a area bounded by Rockville Pike, Old Georgetown Road, Nicholson Lane and the Georgetown Preparatory School. The highrise will be near to the Grosvenor Park apartment complex, which is being totally converted to condominium ownership.

Berger/Berman's partner in the elderly housing proposal, Harkins Associates, Inc. of Silver Spring, submitted a proposal to HUD July 6 for federal funds to pay all rental costs in excess of 25 percent of tenants' incomes.

The apartment rents are expected to be set at about $400 a month, said project director Ed Garbarino.

If Harkins is unable to obtain federal funding for the project, the site will be turned over to the Montgomery Housing Opportunities Commission for development, Maier said.

Planners frequently worry that developers will squeeze too many housing units on a project site, but Peter Berman, president of Berger/Berman, one of three firms building at Timberlawn, wanted just the opposite.

In the meeting of the Maryland-National Capitol Parks and Planning Commission last year in which he gained approval for the project, Berman said he wanted to build fewer than three units an acre. He could have built twice that many.

Commission Chairman Royce Hanson said he was "disappointed" in the low density because of the property's proximity to the future Metro rapid rail system. But, he pointed out, "there's no law that says you have to develop to the maximum."

Hanson said he agreed with the commission staff analysis that property so close to the Bethesda-Rockville shopping corridor and the future Grosvenor Metro stop should have higher densities. As many as 915 housing units could have been built on the site.

But Hanson recalled that the property's sellers, Karl Corby and Mary Ellen Corby Brewer, did not want high-density housing. He predicted that high densities would have been opposed by neighboring communities as well.

Berman denies reports that the market for luxury houses is cooling. And he maintains that houses costing $150,000 to $500,000 are not high-priced. The National Association of Home Builders says the average price of new houses in this area now is $90,700.

While Berman says plans for the houses aren't complete, in earlier reports he indicated that some Timberlawn homes could cost up to $500,000 and would include private tennis courts and swimming pools. No public facilities are included in the plans.

The estate's showpiece, the Timberlawn mansion, will retain 1.4 acres. Leased until recently by R. Sargent and Eunice Kennedy Shriver, Berger/Berman has it on the market for $650,000.

The Shrivers recently purchased a house of the late Vice President Nelson Rockefeller on Foxhall Road NW.

Bob Stithens, a spokesman for U.S. Homes, said that national firm will build 273 brick houses at Timberlawn, on lots average 15,000 square feet. The 2,800- to 3,000 square-foot "traditional" houses will sell for $175,000 to $200,000. Model homes will be ready by late summer, Sithens said.

Holland & Lyons, until now renovators and builders of expensive town houses in the District, will build 95 homes on 37 acres. The houses will sell from about $235,000 to $250,000, said vice president John Conrad.