Bertha and Vincent Daly did what other couples do when they reach retirement age and are living alone. They moved to a smaller place - out of a detached house and into a condominium.

But now they have fled the highrise world of Condo Canyon in Alexandria and returned to the subdivided countryside of split-levels in Fairfax County.

Summing up why he and his wife went from Watergate at Landmark to Burke Centre, Daly said: "There's a sense of something being yours. You don't have a common wall with someone else. It's a matter of pride, of owning something that's strictly yours."

The builders in Fairfax County know exactly what Vincent Daly is talking about. In western and southern Fairfax, single-family detached houses seem to be going up everywhere. According to plans already approved by the county government, 70 per cent of all new housing units in Fairfax within the next couple of years will be detached house, compared to the current 51 per cent.

Approved site plans call for nearly 9,500 detached units to be built in Fairfax. These houses will start coming on the market in about 1 1/2 years.

In Montgomery County, detached houses accounted for only 34 per cent of the housing market in 1974, but by 1976 they had a 45 per cent share. That share is expected to hold in 1977, when between 1,800 and 2,000 detached units are expected to be built in the county.

"Logically home buyers should be looking for a more compact house, but that's not what they want," according to Susan Matlick, executive director of the Suburban Maryland Home Builders.

"They want something that's more open, more individualized . . . I am surprised that people are willing to sacrifice everything they have and put it into a house. But that's what they're willing to do. It's not just a dream, but it's hard reality," she said.

What's happening locally is also true nationally, according to the National Home Builders Association. Detached houses "are just what buyers are buying," said one association official. Despite their higher cost, detached houses are popular, even among young families, the official said, because "there's a lot more double incomes and a lot more mortgage money avialable at lower interest rates."

The trend was supposed to be the other way around. In the 1970s, the detached house has sometimes been compared to the gas-guzzling car. It is supposed to be a waster of land and energy. With the cost of shelter rapidly outstripping income, it is the least affordable kind of dwelling. But despite all of its alleged shortcomings, the detached house is enduringly popular with buyers.

The Dalys, perhaps, are an exceptional example. More typical are couples like Bob and Janet Rust, who now live in a town house in the Braddock Lake subdivision in western Springfield.

"I am tired of the squashed-up feeling you get in a town house," Mrs. Rust said. "I want something more unique, something, I guess, more elegant."

The Rusts have bought a detached house, the Valley Forge model, in a Ryland development in Burke Centre, a new town in southern Fairfax that has gotten off to a quick start selling single-family homes exclusively. They hope to move in some time in June.

In their Valley Forge, which cost them about $56,000, the Rust will have both more and less than they now have in their town house. Instead of three bathrooms they will have one and a half. The kitchen will be smaller. There is no garage. However, there will be a family room they don't now have, plus an unfinished room that could be made into a fourth bedroom.

Compact, equipped with fewer frills, built on a small lot (6,000 to 7,200 square feet or less than a sixth of an acre), the Valley Forge is one reason there is still a broad market for detached houses, why many of them are competitive with town houses.

The Valley Forge is also less of an energy glutton than older detached houses. It has 12 inches of insulation in the ceiling, thermopane windows, fewer exposures, and a heat pump that can save up to 40 per cent of heating costs.

According to the Potomac Electric Power Co., a detached house of about the same size as the Rusts' (1,200 square feet) and with all the energy-saving features would cost about $385 to heat and cool for a year. A same sized typical town house (no thermopane windows, less insulation and individual room thermostats but no heat pump) would cost $508 to heat and cool.Equipped with a heat pump, the same house would have a yearly heating-cooling bill of $375, only $10 less than a detached house of the same size.

In the early 1970s town houses were pushed as a more economical, more environmentally sound type of housing. Sharing side walls, they were cheaper to build. Clustered close together, they saved land that could be set aside as community open space. Or so the theory went. But it didn't always work out that way.

More restrictive fire codes, plus other building requirements, helped to drive up the cost of constructing town houses, virtually erasing savings. At the same time, some detached-house builders were developing cost-cutting techniques, such as partial prefabricating.

Thus, while the median price of housing in Fairfax almost reached the $65,000 mark this year, Ryland was able to offer detached models - like the Valley Forge at Burke Centre - for almost $10,000 less.

David Miller, who heads Miller & Smith, Inc. (he is also president of Northern Virginia Builders Assocation), said his building firm "tried to get our town houses down to the lowest price we could. That was $48,000. But we were building single-families for only a couple of thousand dollars more."

Besides using some factory techniques, builders of detached houses have also been able to hold down costs by taking off their developers' hats and sticking to construction. No longer do they buy large tracts, paying for vacant land while it makes its way through the slow rezoning process and other steps in the costly development process.

Instead, builders tend to buy finished lots - and then only after a homebuyer has signed a sales contract. Their carrying charges (interest on loans) are thus minimal.

Builders also use financing to attract buyers who are on the borderline of eligibility. For example, Ryland (which has 18 locations in metropolitan Washington), by purchasing big mortgage commitments from lending institutions, is able to sell houses costing as much as $80,000 for as little as 10 per cent down, and at a favorable 8 3/4 per cent interest rate. Banks usually won't accept less than 20 to 25 per cent down on houses in the higher price range.

Easier financing, together with the increasing phenomenon of the double-income family (the Rusts are an example), has helped to keep up the sale of even the expensive single-family houses that were supposed to have been priced out of the market.

But sometimes families can buy expensive models even when they don't meet traditional income requirements (mortgage payments should be no more than a quarter of income, lenders say).THey can do that if, like Charles and Claudette Reynolds of Wakefield Chapel Estates, they have $40,000 or more of equity in a present house.

The other Sunday, the Reynolds (he is with the Corps of Engineers in the GS 11, $17,000 to $22,000 salary range, and she doesn't work at this time) were looking at a $90,000 house in the William L. Berry Co.'s Woodberry Burke Centre.

According to the rules, the Reynolds would not be able to afford a house costing more than $53,000. But if tey sold their present house for the $82,500 that they say is now the market price, they could make a downpayment of about $45,000 and manage the monthly payments on the $90,000 house.

When suburban sprawl was under attack in the late 1960s and early 1970s, detached houses were indicted for the amount of land they consumed and road networks they required.

But Fairfax officials say they are virtually helpless in reshaping the housing market, pointing out they have trouble enough in establishing zoning districts.

For the county, the continued popularity of detached houses may bring what all the planning has largely failed to do - slow down the rate of growth.

Although they consume more acres than town houses, detached houses mean lighter densities on the acres. Detached house used to mean a lot of children, but builders report that couples with one child or none are buying three- and four-bedroom houses, often as an investment.

"It's an American dream," zoning attorney John T. Hazel Jr. says of the single-family detached house. "The builders have tried triples, piggybacks, duplexes, triminiums, patio houses. But buyers always go back to the single-family." Like Bertha and Vincent Daly.