THE 13 OAKS, one from each of the original states, saplings when President James Monroe planted them, are thick and old now. The wings of the house have spread to a grander width than Monroe could afford. Cars, not carriages, pull up the winding, tree-guarded drive and the Concorde splitss the heavens above, where once only crows disturbed the peace.

Oak Hill, almost alone among the great mansions built by the early Fathers of the Nation, is still a private gentlewoman and man's countryhouse. Gene and Joseph Prendergast love it and live in it in a manner whose elegance belongs more to Monroe's day than ours.

The 1,200-acre estate wraps the house in serenity like a feather comforter. The house still works as the center of the plantation, serving its purpose as did the country houses of England, where the Virginia gentlemen gained their ideas.

The seven tall pillars of the portico rise to a lordly 30 feet or more to support the great Palladian pediment, just as Thomas Jefferson designed it for his friend. Some say that the five pillars across the facade stand for the first presidents of which Monroe was the fifth.

Mrs. Prendergast points out that Jefferson always like the asymmetry of five columns, and so drew his first plans for the University of Virginia. In the archives of the University, Jefferson's letter to Monroe can be read, promising more extensive plans.

The columns are all you glimpse through the trees, as you drive down Route 1, near Aldie, Va., not far from Middleburg. First comes the historic highway marker, and then just a flash of the white columns before they are screened by the big, old trees.

We parked at the north entrance to the house, a correct Georgian front, with the obligatory boxwood bushes. The graceful double stair leads up to the ship's bell, a gift from the crew when the USS Oak Hill was decomissioned. The shuttered door, with its splendid fan light, is recessed inside a thick brick archway, protecting the door from the north winds.

The house originally had a north portico here. According to Mrs. Prendergast, "I think Monroe said, 'Let us have a portico and spare no expense.' Henry Fairfax, then the owner, had it removed because the farmworkers with problems would come and sit on the steps until he'd go out the door and then they'd pounce on him."

The house's brick, fired on the land, is in two shades, so you can trace repairs and additions. Once the house was painted an unfortunate yellow, but the Prendergasts had 13 layers of paint cleaned off -- it took a year and a half . . . "only to see that some brick is pink and other red and the chimneys yellow," Gene Prendergast said, as she showed us the exterior. The mellow natural look to the brick is far nicer than paint would be.

The front door leads to a proper front hall, built according to the enlightened energy-efficient rules of the period, with an airflow to the garden side of the house. From the front door, you can lok back over a perfect Indigo-Jones landscape of rolling meadows and a lake. "Years ago," Gene Prendergast said, "a photographer took a picture from this door and touched it up to look as though there were a lake in the landscape. Several years later, we put in a lake."

On the portico, on this brilliant fall day for which it was made, the Prendergasts and their guests sat on wicker chairs and ate garlic-spiced cheese cnapes with their drinks. The estate and Mrs. Prendergast's garden stretched out below in bucolic beauty. The portico, as is fitting to the Palladian principles, sits a story up on an arcade, the better to survey the kingdom.

The Prendergasts told one of the house's great stories. During the War for Southern Independence (as they would have called the Civil War in Virginia), Union Gen. George C. Meade installed himself, without invitation, in Oak Hill, to the distress of Mrs. John W. Fairfax, owner of the house and a Southern sympathizer. One afternoon, as he sat on the portico, a swift horseman galloped across the fields. Some time after, Mrs. Fairfax said, casually, "That, General Meade, was Colonel Mosby." The Gray Ghost of the Confederacy of the Northern troops, had escaped the Yankees once again.

Another visitor was more welcome. The Marquis de Lafayette visited twice.In August 1825, Jane McIllvaine McClary (writing in "Virginia Country" in the winter of 1980) wrote that de Lafayette, "President [John Quicy] Adams and former President Monroe led the entourage to Leesburg in a carriage drawn by white horses."

Monroe first lived in a farmhouse on the estate. This small house, as Prendergast explained, "has been stabilized, but not restored. We aren't too sure what we want to do with it." Mrs. Prendergast said, "We keep our spooks in it." Monroe built the RomanRevival mansion as a summer White House. The house was finished in 1823, but he moved there permanently after he finished his term of ofdfice in 1825.

The estate almost bankrupted Monroe, because he tried to keep his standards of Southern hospitality in the presence of hordes of visitors. And it did bankrupt Frank C. Littleton, who owned it until 1948 and claimed he would make a national monument of it, only sadly to have it slip through his hands at the end of his life and money.

The original house had much smaller, rather unfortunate extensions that looked more like lean-tos than proper wings, likely because Monroe ran out of money after the majestic central portion was completed.

A hundred years later, in 1924, the house was completed finally, when Littleton enlarged the wings by 15 feet on the east and west, to the thoughtful and correct design of architect Henry D. Whitfield.

Gene Prendergast and her late husband, Arlington builder Thomas DeLashmutt, bought the estate at auction in 1948 for a bargain $220,000. After DeLashmutt's death, his widow married Joseph Prendergast, then executive director of the National Trust for Historic Preservation. He has since retired, but Mrs. Prendergast is a board director of the Historic Houses Association of America. Ties to the White House

James Hoban, the winner of the competition for the President's Palace, built Oak Hill, using many of the same workmen who rebuilt the White House after the fire during the War of 1812. Over the years, there have been suggestions that some of the materials from the White House found their way to Oak Hill. Admittedly, the flooring in part of the basement came from the renovation of the White House. Some of the iron railing is certainly similar.

The flagstones in the garden and other parts of the basement still carry the three-toed prints where dinosaurs walked millions of years ago. Their origin is more certain. They come from a quarry on the property.

White House curators over the years have tried to lay claim to the marvelous white marble mantels in the east and west parlors. Owners of the house have always steadfastly maintained that the mantels were the gifts of the Marquis de LaFayette, in appreciation of Mrs. Monroe's influence in saving his wife from the guillotine.

In a news story by Chambers M.Roberts in 1949 in The Washington Post, Laurence G. Hoes, a direct Monroe descendant, testified that the tradition of Lafayette's gift had been passed down in the family. White House architect Lorenzo S. Winslow, according to the story, claimed that the Oak Hill mantels were two of 14 brought for the White House from England about 1792.

The current official White House guidebook says that Monroe ordered two neoclassical mantels from England in 1817 for installation in the State Dining Room. These two were moved to the Green and Red Rooms in 1902.

The mantels at Oak Hill have a similar acanthus leaf design to the White House's Monroe mantels. Winslow was sure that all four were made by the same 18th-century English carver. Mrs. Pendergast said "Ambassador David Bruce , who then owned Oatlands, our neighboring plantation, told the curator, 'You aren't going to touch Oak Hill's mantels.'"

It is a matter of record that Monroe bought furniture in France for the White House, after the fire, and took it with him when he completed his term. Much of the suite has been returned to the White House.

The house was still not quite finished when Monroe signed the Monroe Doctrine's 36 handwritten pages in his study. This room is now incorporated into the extened west wing, the grand dining room. The Grand Tour

The other day, we dined sumptuously on spoon bread made from homegrown corn ground at the Oak Hill mill, chicken from the Prendergast coops and exquisite tomatoes, picked just at the perfect ripeness from the garden.

On the walls are the famous Zuber wallpaper murals, "Scenic America," used in the White House. As the centerpiece, Mrs. Prendergast used the silver 19th-century monleith, a punchbowl with scallops to hold glasses, recently in the National Gallery of Art's American Decorative Masterpieces show.

After lunch, Mrs. Prendergast gave the grand tour. The two central drawing rooms with their 16-foot ceilings have the famous marble mantels. Mrs. Prendergast has them painted pale violet "because it looks best under candlelight." The chandeliers, filled with candles to turn evening parties into magnificent events, come from a Dupont Circle house.

Much of the furniture here was bought in 1952 at a landmark five-day auction of 700 pieces at C. G. Sloan's. By today's standards, the prices were ludicrously low.

The collection had been owned by Frank C. Littleton over his 30 years of glory at Oak Hill.

The DeLashmutts bought many of the best pieces, including a walnut Chippendale side chair that belonged to Lord Fairfax ($120), antique mahogany bed steps in three tiers "with all the necessary conveniences" ($100). Benjamin Williams, an antique dealer in Highland, N. C., bought 11 pieces. Vice President and Mrs. Mrs. Alban Barkley, according to a 1952 news story, were up front to buy an "antique circular mahogany dishtop table, with bird cage ($85), a heavy Tiffany silver dressing spoon ($20) and two dozen Damask hand towels ($50)," Laurence Hoes, then presendent of the James Monroe Memorial Foundation, bought for $85 a circluar mahogany marble top commonde and a mahogany falling front secretary ($250), both from President Madison's Montpelier.

The gilded pier console is sometimes called a petticoat mirror, because you can see you petticoat in the lower part of it. "When we bought the console at the Sloan's auction for $50," said Mrs. Prendergast, "it was all tied together with a rope, and a marble shelf was missing. We found it in the basement."

One of the chairs belonged to Mrs. Jumel, Aaron Burr's wife. The sofa, originally owned by Madison, was in a garage when Gene Prendergast bought it. A bench came from Gunston Hall. One of two busts, Mnemosyne, mother of the Muses, was auctioned off as the mother of Moses. Gene Prendergast thinks of her as the household goddess, to be decorated at Christmas.

The Madison bedroom, complete with the elaborately carved posts and the original bed hangings (". . . and the original dirt," said Gene Prendergast with a laugh), was bought from Montpelier. "The ceiling fell on them once, but they still survive. I use the room for one-night visitors." Its bath, with elaborate marble and brass fixtures, was the first installed in the house, by a Dr. Quimby in the late 1800s. A Western state governor who stayed in the house, wrote in a story for a hometown newspaper that he slept in the bathtub because the Madison bed was too short.

The 22-by-28 foot library has Zuber paper as well, the one that shows Lord Cornbury, a 1700s Britsh New York military governor, reviewing the troops in drag. Cornbury, an early day transvestite a la Klinger of TV,s "M*a*s*h," was eventually removed from his position.

The comfortable library has woodwork designed by Frank Almirall in 1952, with a bookcase concealing a small bar. The elaborate woodwork was done by Arlington Millwork, once partly owned by DeLashmutt.

On the mantle piece are copper shingles worn by the weather into patterns like Whistler paintings.

On the driveway side of the house is the master suite. More bedrooms and baths are upstairs.

Downstairs, on the ground floor, is the old dining room, kitchen, bacon and ham curing rooms and the servants hall. Much of the big furniture here comes from Mrs. Prendergast's family house near Richmond. Prendergast uses the east room, with it's big original fireplace, for his study. Caring for Great Houses

The road to Oak Hill is almost hidden by trees and bushes, a good thing, too, because the Prendergasts should not be worried with unannounced visitors. tThey are generous about opening the house to arranged tours, (with a fee toward its maintance and the $1,000 January heating bill), but it has to be requested in advance.

Recently, the Prendergasts were hosts to the members of the Historic Houses Association of America, the people who throughout the country are are working to preserve the wonderful houses that have come into our century with their beauty and utility still intact.

The members of the Association who owned houses talked about problems such as how to maintain a three-mile drive way or how to clean 15 chimneys or how it get rid of unwanted visitors. Other members who don't have houses, stood and listened and thought, "I wish I had a house like that. But I wouldn't want the heat bill."

The maintenance of a historic house such as Oak Hill has problems most people never think about. Mrs. Prendergast said last year the chimney sweep counted the house's 15 or so chimneys and said: "Keeping these chimneys cleaned would be a life's work." One former owner had a man who did nothing but wash the oil lamps.

Most of the other great early houses of Virginia, are today owned by foundations and trusts who administer them with great care and industry. The public is privileged to have these houses cared for and open to them.

But at Oak Hill, life is not held back by velvet ropes. Nor does it take a committee to decide whether to buy another Madison chair. Oak Hill has had an uncanny fortune -- a series of hairbreath escapes. Even its furnishings have been lost to the house -- and regained again by the right owner with the right money appearing at the moment when the hammer goes down.

Oak Hill is a bridge between what was and what is. Alive and Thriving, Oak Hill owes its life to people who have preserved it for the future while enjoying it in the present.

Mrs. Prendergast said she's never seen a ghost at Oak Hill, though Littleton is supposed to have seen an early Virginian, John Randolph. "But sometimes," said Mrs. Prendergast, "when it's very quiet, we can tell that people who have been here before us, have left something behind."