The grand manor is no longer in peril of the grim wreckers.

After the fall of the stock market in 1929, such magnificent manors stood in terror of the Devil Developer or the Fluorescent Fiend, coming to slice up their gardens into row-house tracts or screw commercial lighting fixtures into their chandelier medallions to light the multiple offices of paper shufflers. The lucky ones became embassy residences.

But in the last two years, the sales of The Rocks in Crestwood and Dunmarlin on Foxhall Road -- both with sites the size of European principalities -- have marked the arrival of trillionaires with palates for palaces. A week or so ago, Foxhall Road neighbors met to hear Rafic A. Bizri, president of Raha III, reassure them that Rafik Hariri, the Saudi Arabian businessman who owns the company, intends to use Dunmarlin as a residence. "Mr. Hariri has houses in other places," Bizri said. "But he intends to stay in this one when he's in Washington."

Bizri, who smiled a great deal but said little, admitted the property had been bought as an investment. And he made no promises about the 12 acres beyond the house's four-acre site. Still, he did point to a tractor, at that moment busily wiping out the roads that previous owner John Driggs had cut when he planned to sell the land off for million-dollar houses.

Instead, Driggs, who owns a construction company, sold the house (of 26 rooms) and the land (16.2 acres fronting on Foxhall Road) to Raha III for $13 million. Carol West of Michael Sullivan Inc. was the real estate agent. Pat Dixon was the listing agent. The two of them are now very wealthy and very happy with their share of the highest residential sales price ever recorded in the District.

That price is more than twice the record, that of the aptly named The Rocks (seven bedrooms, 15.9 acres), sold in March 1985 to Sen. Jay Rockefeller (D-W. Va.) and his wife Sharon, then a board member of the Corporation for Public Broadcasting.

The other great recent estate sale was Merrywood (26-room house, seven acres of Potomac riverfront on the Virginia palisades, $4.25 million), sold to investor Alan Kay and his wife Dianne in July 1984.

Dixon said the Dunmarlin acreage had actually already had several takers. "We had nine lots spoken for. But John Driggs had been in Europe for a month, so he hadn't accepted them and signed the papers. We tried to sell Mr. Hariri 13 acres, so we could sell off the other three as million-dollar lots. But they wanted the whole parcel, I suppose so they would be sure they were protected."

Across the street from Dunmarlin are the 2 1/2 acres Alan Kay put up for sale when he and his wife decided to buy Merrywood.

"It {the old Kay place} could have seven houses," said Dixon. "But the new owner -- I can't tell you the name because the settlement isn't until August -- is going to put up just a single house for his family, not pretentious, right in the middle of the site. The asking price was $2.2 million."

Meanwhile, real estate people look with yearning at three other great Foxhall estates not likely to be up for sale anytime soon: Elinor R. Brady's 16-acre estate, with its chateau, (in 1985 the highest assessed residential property at $5,255,157); David Lloyd and Carmen Kreeger's art-filled Philip Johnson-designed masterpiece on its 5 1/2 acres and Gwen Cafritz's art deco showplace on seven acres next door.

Foxhall Road, with its embassies and millionaire neighbors, is the location, location, location -- said to be the best determinant of real estate prices. Washington, unlike London, Paris or New York, is fortunate in still having multiacre estates within the city limits. But prices go up considerably for a house, preferably with a name (The appellation of The Rocks certainly didn't hurt its sale), a history, and a socially acceptable previous owner.

Other necessary appurtenances:

Several acres of land, so that if you don't invite all your neighbors to your party, it isn't as obvious.

Library, with safe installed (and room for a paper shredder).

Morning room, where madame and her secretaries calligraph thank-you notes and invitations, and cook presents the menus for the day.

Dining room that seats 24 (minimum), so the hostess can invite the ambassadors of the major countries and their wives all at once.

Kitchen large enough so the head chef and the pastry chef won't fight when cooking for 24 seated, 500 for receptions.

Eight bedrooms with at least 10 baths so no one has to stand in line.

Servants' wing for at least six with apartment over the garage for the driver, all plush enough to attract the hard-to-get trained staff.

Guest house, with at least two bedrooms and kitchen -- could double as a bath house, if the price is right.

Clay tennis court, with careful perimeter plantings.

Swimming pool (black surfaced, of course, nothing gaudy such as a blue pool, which architect Hugh Newell Jacobsen likens to a girls' school gym).

Terraces, rose gardens, woods, gazebos, follies, etc.