Builder Patrick Cullinane betrays no anxiety about the fact that the $2.1 million Georgian mansion he completed three months ago is still unsold. The property, high above the Potomac River on Chain Bridge Road in McLean, will sell, he says.

"They always do," he maintains. "Someone will appreciate this house and have the money to buy it -- even in these times. Financing will be no problem for the buyer, who probably will be someone special."

Cullinane, 48, put a lot of himself -- along with 110,000 specially made bricks and eight fireplaces -- into the 18,000-square foot, four-level house he has named "Truro." It had its creative antecedents in old castles that Cullinane studied in Ireland and England.

Just don't make the mistake of calling Truro a Williamsburg-style house. That upsets Cullinane. He dug up three books on architecture to prove his Georgian point, including Nathaniel Lloyd's "History of the English House."

Architect Wilfrid Worland, who has done the architectural drawings for all of Cullinane's expensive, speculative houses over the past 20 years, says the design ideas are all the builder's.

While Worland thinks Cullinane went "overboard on ornamental plaster" for his latest house, he also believes that the builder "produces spectacular results in a quiet way. The dining room (complete with fireplace) is my favorite."

The $2.1 million house has a marble foyer with columns, a two-level "grand room" with floor-to-ceiling windows in 15 sections. The beautifully balanced front facade has understated elegance; the slate roof is broken by five delicious dormers.

The house is set on more than four acres, including 360 feet sloping down to the river. It is set off by a circular driveway with a clump of imported old boxwoods (another Cullinane trademark) as the centerpiece.

Truro is Cullinane's first house in Virginia. Most of his 56 expensive houses have been built in Potomac or in the Burning Tree area, where he and his late father, Leo (who died last year), began building speculative houses in the 1950s. Cullinane has four brothers, all of whom are in real estate-related jobs. One brother, Anthony, recalls that the Cullinanes were building custom houses for $78,000 in the 1950s -- when that price scared away lenders.

Leo and Patrick Cullinane had previously built more conventional, small-family dwellings in Wheaton. Patrick, his mother and brother Robert still live in the two-story colonial that the Cullinanes built years ago on Westway Drive in Bethesda.

A friend and former client, whom he would not name, was the owner of the Chain Bridge property, Patrick said. It was the "last remaining big riverfront site in the area," he said. "I looked at it an decided it could be the site of a truly great house . . . ." He finished the house in 14 months.

It looks like it has been there for decades, the house of a wealthy person with a penchant for entertaining on a grand scale. The house has seven bedrooms, seven baths, an elevator, an oak spiral staircase to the library and twin front stairways. There are four garages and two heating systems and two cooling systems.

If you have to ask about the taxes -- or what it will cost to heat and cool the place -- Truro is for someone else.

What about the neighbors? Sen. Edward Kennedy (D-Mass.), whose plans for 1981 include a move into town, lives in a contemporary house to the east. Truro overlooks the Kennedy pool. To the west, there's the Williamsburg-style, steel-and-concrete castle that Louis Pomponio built (and never quite finished inside). It is now listed for $2.2 million. Young & Simon Inc., an insurance firm in Georgetown, is the owner, according to real estate broker Joan Day.

Harper & Co., a McLean real estate brokerage firm, has the listing on the Cullinane house and arranges showings for serious lookers. Spence Rivett is Harper's agent in charge. Like Cullinane, she is confident that the house will sell.

"There may be only a few dozen potential buyers, but the right one will recognize the value and grab it," she said. "We've have some offers and nibbles but nothing really serious to date." CAPTION: Picture 1, A rear view of the new $2.1 million Truro mansion on Chain Bridge Road in McLean; Picture 2, interior of its "grand room," highlighted with 270 window panes. by Larry Morris -- The Washington Post; Picture 3, Georgian mansion was styled after castles builder Patrick Cullinane found in Ireland and England. By Larry Morris -- The Washington Post; Picture 4, Cullianane descends from library to "grand room." By Larry Morris -- The Washington Post