A sales office has been opened in the gatehouse of the Hillandale estate west of Georgetown and if you have $3 million to spend on a house, the 48-year-old Archbold mansion there is for sale.

Also for sale on the Reservoir Road estate are town houses beginning at $275,000, which a TSI Corp.-related development firm plans to begin building soon. The top pice anticipated for the town houses is $450,000.

The sales office-gate house has been fixed up by the developer, Hillandale Development Corp., Inc., which is part of the Dallas-based business of the C. W. Murchison family.

The development firm earlier reached a 99-year, land-lease agreement for more than $1 million with property owner John Archbold, who still lives in the 16,000-square-foot Florentine villa on the property that was built by his mother, Anne Archbold. Michael Gulion, vice president of the Hillandale development firm, said that Archbold plans to recreate some of the Hillandale rooms in a country place he is currently enlarging in Upperville, Va.

Eventually, a new entrance to Hillandale will be constructed on 39th Street (the eastern boundary) and the present gatehouse entrance will be closed.

In addition to the town houses, there will be 28 7,500-square-foot sites at for detached houses. The losts cost $185,000.

Plans for the detached houses will be reviewed first by a group headed by architect Alan Lockman, whose Washington firm designed the Hillandale town houses.

Lockman is also the architect of the, $50,000-plus detached houses that developed Robert T. Foley is beginning to build nearby at Reservoir and Foxhall roads. The land was formerly owned by developer Waverely Taylor.

Up Foxhall Road, developers Alan Kay, Buddy Rozansky, William Bush Jr. and Stephen Gallin have been preparing sites for 125 houses on the 25-acre Rockefeller tract. That's where Crowell & Baker plans to build Georgian houses styled by architect Arthur Cotton Moore for buyers who don't blink when prices of $350,000 or more are mentioned. Initial construction is expected to begin this spring or early summer. Pre-sale deposits already have been taken from more than a dozen prospective buyers.

Architect Lockman also is the designer of the Westover Place traditional town houses that Kettler Brothers Inc. has been building for a year at Massachusetts and New Mexico avenues NW.

Gulino said he regards those Kettler town houses and some other smaller developments in Northwest as "competition" for the first houses being built at Hillandale. Gulino also said that preliminary purchase agreements with $10,000 deposits have been signed by persons interested in the firsst 26 dwellings to be offered. CAPTION: Picture, the 48-year-old Archbold mansion is for sale on the Hillandale estate west of Georgetown. The price is $3 million. By Margaret Thomas -- The Washington Post