The day after Paul and Margaret Glenn went to settlement on their new $97,400 house in Annandale, the builder came to them with an offer: Would they sell back their house for $115,000?

The Glenns could have made an 18 percent profit in one day - a rate of return that would dazzle even the gnomes of Zurich - but they said no, thank you.

After all, they had just turned a $38,500 profit on their old house in North Arlington, and the new place, on Bennett Drive, had the extra space and larger lot they wanted.

Besides, it probably would not be an eternity before the house on Bennett Drive would be worth a lot more than even $115,000. The price of similar, only slightly larger houses still under consideration in the Glenns' subdivision, Bennett Estates, is in the $135,000 range.

In an admittedly exaggerated sense, the Glenns' story illustrates how a sizable number of middle-class Americans have manage to skim along safely in the curl of the inflationary waves that have been battering the economy for 10 years.

They have done so by well-timed decisions to buy a house, sell it, then buy another - often making whopping down payments of 35, even 40 and 50 percent to offset higher interest rates. For them, buying a house has become an investment that outper performs the stock market, savings accounts, even the market for gilt-edged bodns.

"A houseis probably one of the best investments you can make," said economist Dennis Jacobs of the U.S. League of Savings Associations. "Gold is perhaps better. But you can't sleep in gold."

Although statistics are unavailable on how many investor-homebuyers there are, a spokesman for the Northern Virginia Board of Realtors said residents here change homes more often than the national average of once every 11 or 12 years.

"In this area," he said, "from age 35 on up, where people have job security, you find moves occur every three to five years, especially since 1973, when people found their properties were appreciating at a higher rate."

Fairfax County has the strongest real estate market in the metropolitan area and one of the strongest in the nation, counting both new sales and resales. While construction of new houses has slowed in the Maryland suburbs, resales have remained strong. In a few District neighborhoods, old houses have appreciated 100 per cent in three years, and in 14 neighborhoods, the appreciated rate was 75 percent.

Not all of this new breed of investors have done as well as the Glenns, who bought their old house on North Florida Street in Arlington for $27,000 in 1972, put in $15,000 worth of improvements and sold it in June for $81,500 - an average annual return of 15.6 percent.

But many of them, according to an analysis of 22 transactions involving recent home sellers in Fairfax County have been able to stay ahead of inflation.

Their annual return was 12.7 percent - a rate well above the 10 percent average annual increase in the cost of housing.

One example: Frederick and Mary Ashley bought a house in McLean for $87,000 in June. Though the seller made a $53,500 profit on the house she had bought for $33,500 in 1967, the Ashleys did not lose any of their purchasing power to inflation. They made $48,000 on the house in Arlington they had purchased for $37,000 only five years ago - an average annual return of 25.9 percent.

The Ashleys' monthly mortgage payments went from $290 to $485, but a big chunk of that difference will come back as refunds on their state and federal income taxes. In addition, they have the extra space that would have cost them $30,000 if they had built an addition to their old house.

Furthermore, they now own a house in a prestigious area of McLean, Broyhill Glen. In less than two years, the value of their house - if the current appreciation pattern of 12 to 16 percent annually holds - should rise to $100,000 or more.

"If we hadn't owned our first house," Ashley said, there's no way we could afford this one. All we would be able to buy otherwise is something costing $50,000."

Like other homeowner investors, Ashley has been able to ride the inflationary wave because he and his wife decided to buy a house - in 1973 - before the escalation of prices reached its current rapid rate.

Yet another incentive to trade up is the federal tax law, which permits a homeowner to keep all the profits from a sale if he buys another, more expensive house. This capitals gains benefit would become even more generous under tax law revisions passed by the House and now before the Senate. Under the changes, a homeowner could make a one-time profit of up to $100,000 even when he does not buy another house.

In nine years, Gerald and Christine Partch have been able to go from a $21,400 house they bought in Dale City to an $81,995 house in the shiny new town of Burke Centre in Fairfax's rapidly developing Pohick area west of Springfield.

This is how they did it:

They sold their first home in Dale City for $28,500 in 1972, put their windfall of equity into a $33,500 house, also in Dale City, and sold that one for $51,000 this June.The cumulative profit became the 30 percent down payment they put on their house in Burke Centre.

"It's like playing the market," said Partch, a little amazed at the spiraling figures. "But you don't have to float around a large amount of money. The money doesn't come out of your pocket."

Time and again there is the note of amazement that everything is going exactly right.

"I can't get over it," said Carol Eberhart. She and her husband, James, have in nine years, catapulted from a $16,000 house in Norfolk to a $77,883 house in Burke Centre.

"We're planning on making a great deal on this house," Mrs. Eberhart said. "We're hoping to go to another area in about eight years and pay cash for a house."

The Eberharts' prospects look good because (1) they bought a new house, which generally appreciates faster than an old one; (2) they chose a single-family model, which appreciates faster than a town house or condominium and (3) they moved early on to an area - Burke Centre - whose image is matching the current suburban dream of a touch of "down home" with all the modern conveniences.

Investment-minded homebuyers also stress the importance of well-timed selling. Says Gerald Partch, who has sold twice: "We sold our homes in Dale City when we felt their appreciation had about peaked."

For years, homebuyers have been solemnly told that their purchase was the most important they would make in their lifetime. But for some veteran property owners the experience is no longer such an awesome undertaking.

Army Lt. Col. James McCune and his wife, Gale, recently moved into an $83,000 house that Mrs. McCune bought while her husband was still at his post in Korea. Less than two months later, the McCunes already have their eye on a $91,000 house that they think suits them better.

Though they have only been in their house since June, McCune thinks they can sell it at a high enough price to recover closing costs.

McCune, who has taken sizable chunks of equity from two house sales, thinks the boom that has benefited him and other homeowners will continue. "We're running out of property, we're running out of space," he said. "Investment in real estate is foolproof. It's the best."

While homebuyers who got in on the ground floor are often reaping equiety windfalls, newcomers to the market often have to pay more for less.

For example, Air Force 2nd Lt. Mark C. Pavlin and his wife bought a $58,950 town house in Burke Station Square through a no-money-down Veterans Administration loan. Their monthly payments are more than what the Glenns, who live in a $97,400 house, pay.

But Pavlin said: "We decided to buy a house at all costs. You have to get your foot in the door some time."

As investment-minded as other buyers, Pavlin said his real estate agent told him that in six months his town house should appreciate $5,000. "That's what the computer said," Pavlin said.

David E. Pickering II was one of the increasing number of middle-class Americans who were priced out of the bullish housing market not because they could not meet the monthly mortgage payments but because they could not raise the now-hefty down payments required for conventional loans.

The the FHA raised its borrowing limit to $60,000, and Pickering was able to find a qualifying $53,500 house for which he had to put down only about $1,000 on an FHA-guaranteed loan.

"I was forced into it," Pickering said: "I looked at my salary check, and I was paying more and more to Uncle Sam and had nothing to show for it."

Already Pickering is beginning to think as an investor. About his town house, which he has owned for two months, he says: "It has a lot of potential. If I do this and do that, I could get a lot more." CAPTION: Picture 1, In nine years Gerald and Christine Partch were able to move into an $81,995 house, where they are pictured, By James A. Parcell - The Washington Post; Pictures 2 and 3, by selling their first home for $28,500, (at left), and moving into the house at right, which they sold for $51,000, By James M. Thresher - The Washington Post