Nearly one-third of the lots in what is being billed as the first equestrian community in Maryland have been sold since last month.

Hollow Tree Farm, a 150-acre partly wooded site in Olney, has been carved into 30 home sites of at least two acres each, bordered on three sides by 86 acres of open common space to be devoted to horses.

The development has been laid out with equestrian needs in mind: Ten-foot, soft-surface easements will run along the asphalt roads to allow horses to be walked, several paths give horses access between lots to the surrounding common area, and an equestrian right-of-way leads from the common area across a county road to 20 miles of connecting bridle paths. Even the decision to divide the site into two-acre-plus lots took into consideration a Montgomery County rule that a resident of this area of the county can have a horse "reside" on his or her property if it is at least two acres without applying for a special exception.

The lots--there are 29 home sites from two to 2.88 acres and one that is 4 1/2 acres--are being sold at prices ranging from $41,000 to $58,000. Hollow Tree Farm is situated off Old Baltimore Rd., minutes from Georgia Avenue.

The last lot is priced at $285,000. It already contains a 23-room, seven-bedroom refurbished Colonial house--part of it dates back to 1830--and five outbuildings. According to the house's current tenant, the house and grounds were used by members of the Carter administration for numerous Georgia-style barbecues.

The equestrian development is the brainchild of Martin Seldeen, president of Seldeen Development Corp. and part owner of the property, who said he spent much of the last 16 years since purchasing the land trying to get it zoned in such a way that he could develop it. The property was designated as a green, or rural, area, and area citizens groups opposed Seldeen's initial efforts to gain zoning to divide the property into half-acre lots.

One area resident's statement at a meeting that neighbors had come out there to live in a rural area--and have their horses--gave Seldeen the idea for an equestrian community, he said. Work with the County Planning Board to devise a plan that would satisfy both the developer and the citizen groups resulted in the creation of a "rural cluster zone" for the area. That requires it to retain some of its rural character when developed as a residential area.

Under the zoning, Seldeen could group the home sites on a smaller portion of the property. The overall density requirements remained at one lot for every five acres of the total acreage, while the clustered home sites could have been a minimum of one acre so long as 60 percent of the total acreage was preserved for agricultural or open space. Seldeen opted for a two-acre minimun lot size.

The 86 acres, to be owned by the homeowners association, could be used for stables, riding rings, jumping courses, grazing areas or left open--whatever a majority of the participating owners decide to do, Seldeen said.

Under the by-laws, owners of lots who don't want to participate in the common area can't be assessed for any projects there, according to Robert T. Baum, executive vice president of DiSalvatore Realty Inc., which is selling the lots. (Not everyone who bought a lot so far has a horse, he noted.) Lot owners also can decide to do something like build a swimming pool or tennis courts, but that decision would require the vote of a majority of all the owners, Baum added.

All of the lots will have public sewer and water, and the roads in the development--Palimino Court, Quarter Horse Way and Covered Wagon Way--will be maintained by the county.

Right now, contractors for the Washington Suburban Sanitary Commission are laying storm sewers on the land--once used for cows and corn--and underground utilities are being installed. Ed and Carol Ames' newly purchased lot carries a hand-lettered sign asking workers to please be kind to our trees . . . These trees are special to us--they are why we bought this pretty lot."