A Gaithersburg development firm said yesterday it intends to build a 2,200-home planned community and a massive international marketing mall on the outskirts of Leesburg--a project that, if built, would increase the rural town's population by one-third.

George Kuchta, president of Leesburg Properties Inc., said the centerpiece of the proposed development will be a 750,000 square foot neo-gothic "crystal palace" that will contain pavilions, stores and restaurants operated by foreign governments.

But Leesburg town officials say they have only seen a preliminary concept plan for the first phase of the complex, a plan for a strictly residential development, and that they do not know what plans for the international marketing mall will be.

"The concept plan for the residential community is a good one," said town manager John Niccolls. "It's more creative than most of the ones we've seen. But we know nothing about the concept plan for the business or marketing center."

The venture is being financed by the state-chartered Old Court Savings & Loan in Baltimore and other private entities company officials declined to name.

The first phase, scheduled to start within two months, would be 1,300 homes, including single family detached houses, town houses, duplexes and condominums, built on a 400-acre tract north of Rte. 7 and just inside the Rte. 15 bypass around Leesburg.

The second phase would include a 300-room hotel and conference center, 150,000 square feet of office space, and the 750,000 square foot enclosed international marketplace, alongside the Rte. 15 bypass. Phase two will take up 72 acres of land and include preserving and landscaping the ruins of "Exeter," an 18th century mansion gutted by fire two years ago.

Phase three would be between 400 and 600 additional homes, on a 400 acre tract just north of the planned international marketplace and would be built last. Company officials said that the entire development is projected to be complete within seven years and that the international marketplace might be open as early as late 1985.

Kuchta said that the company owns the land and has the zoning for the first phases, but that they are still negotiating on the land for phase three.

However, Thomas Nalls, a Leesburg attorney who represents the owners of the tract targeted by Leesburg Properties Inc. for their third phase, said there are no negotiations going on to sell the land to Leesburg Properties Inc. and that the owners are working on a residential development plan of their own.

County officials also expressed surprise at the magnitude of the proposed development, saying that the traffic impact on already-saturated Rte. 7, could pose problems.

Niccolls, Leesburg's manager, said that most of the zoning for the development was already in place and that there would not be a public hearing if the developers did not ask for any major zoning changes. The phase one plan is scheduled to come before the town council in the next few months and construction on an initial 20-house section could be started as soon as it is approved.

Officials with Leesburg Properties Inc. said they were attracted to Leesburg because of its proximity to Dulles International Airport and the possibility that the airport may become a foreign trade zone within the next year.