As the so-called baby boom generation falls behind its parents and even older siblings in lifelong earning power, builders across the United States say they are feeling the impact in the kind of housing these baby boomers want and can afford.

A recent survey of 750 people from 15 metropolitan areas done by Multi-Housing News showed that buyers are more willing to accept smaller living quarters now than they were last year and that interest in single-family homes is declining.

Although the most popular home size is 1,000 to 1,500 square feet, according to the survey, the number of people interested in units smaller than 1,000 square feet rose from 15 percent last year to 24 percent this year. The median square footage preferred for a detached home was down nearly 500 feet from last year. For comparison purposes, the average suburban two-car garage is about 450 square feet.

Builders say they are not surprised to hear about the changing desires of the buying public, and they are beginning to offer more compact units every year.

Robin Neill, a sales agent for Fairfield Homes in Reston, said that two of the more popular series of detached houses developed by Fairfield this past year were homes that ranged from 1,500 to 1,900 square feet, had three or four bedrooms and 1 1/2 or 2 1/2 baths. The houses, she said, were priced between $115,000 and $135,000.

"They are a terrific starter home," said Neill. "People are looking for quality, but not everyone makes $80,000 a year, so we have to offer people options."

In one of the series offered by Fairfield the houses are built on lots of an eighth of an acre or less. Neill said the houses sit on one edge of the lot, with privacy fences instead of side-yards on that lot line. Such so-called zero-lot line development increases density and lowers land costs.

"The Old Westbury surprised us by doing very well," said Neill. "It appeals to singles, young couples and even elderly people who want space for planting but don't want the responsibility of a big lawn."

Kettler Brothers, the developers of Montgomery Village, said they had introduced a similar series this year, a line of detached houses that start at below $120,000.

John Gornall of Kettler's Montgomery office said that to suit the "new tastes of the baby boomers," Kettler had expanded footage by a couple of hundred square feet over last year's models and redesigned the houses so that they had "spaces that people perceive to have more space." Such space-enhancers, said Gornall, include bay windows and Victorian towers.

To make houses more affordable, Fairfield offers amenities as options rather than building them into every home. Neill said that by allowing the second full bath as an option in the Old Westbury series the developer was able to offer a scaled-back detached house that is, for Reston, priced to be competitive with many town houses.

Fairfield officials said that they were even starting construction on a series of detached homes just 1,000 square feet in size, something almost unheard of in the past. The houses have two to three bedrooms and 1 1/2 baths and will be offered in Montclair community in Fairfax County at about $88,000.

Builders say that young people seeking smaller houses are people who may have begun to realize that, with the double pinch of high interest rates and expensive housing, they need to lower their expectations. Builders also say that those most realistic about the market and what they can afford are people who are serious about buying and who have been looking for a house for several months.

As some of the baby boomers have begun to lower their sights, many have opted for condominiums and town houses, and trends indicate that the percentage of new building that is multifamily probably will increase significantly over the next 10 years.

Herve Kevenides, an economist with New York's Chemical Bank, predicts that multifamily units will make up 43 percent of total housing starts by 1995, up from 35 percent in 1983. Kevenides said he defined multifamily as apartments, condominiums and town houses. While cooperatives and condos account for about 2 percent of all the housing stock today, Kevenides predicts that the number will rise to 9 percent of all owner-occupied units by 1995.

"There are a number of reasons for this, one being that condos and town houses are far cheaper to build," said Kevenides. "We are also seeing the inner suburbs of many of the big cities, suburbs that were bypassed by developers in the past 20 years, beginning to fill up again as developers find they are ideal for multifamily building."

Many builders say they are finding that buyers are much more interested in the amenities of a house and community than the size of the unit, and are therefore willing to buy smaller houses if they have the amenities they want. But Kevenides said that that trend has been partly responsible for the increased cost of detached housing.

"In the planned communities now popular in the suburbs, the cost of building roads and sewer, providing pools and parks is now being paid by developers instead of by local governments as in the past," said Kevenides. "Since detached housing uses so much land, this has made detached housing more expensive."

Whether the baby boomers will stay in their condominiums and small town houses for long, however, is something developers say they are having trouble predicting. Some say that the affluent upper third of the group will fuel a wealthy move-up market, while others think that many will end up in their small units for longer than they would like to.

As many baby boomers find they cannot afford to buy their own homes, an increasing number are turning to renting single-family detached houses. In the process they help out their more affluent friends, particularly those who want to move up and out of a town house or small detached house but are having trouble selling the unit in today's high interest rate market.

Eugene Isaacs, president of Epic Realty Services, a Falls Church company that manages single-family rental property all over the United States, said that he has seen a big boom in single-family rentals over the past few years as more and more families choose to keep their starter home as a rental property when they move up or families choose to rent while waiting for interest rates to come down.

"This gives young couples the opportunity to live in middle-class America without necessarily being buyers," said Isaacs. "The people who rent the properties we manage are not your typical renter. They average three or more people per family, have incomes that average about $40,000 a year, and 50 percent of them have owned a house before."

Isaacs said that he believes the people who have owned but are not renting are usually either families that have moved to a new town and cannot afford to buy at today's high interest rates or people whose families have split up.

Danny O'Sullivan, vice president of Long & Foster's new home and condominium division, said that in a survey of 200 renters he found that the average first-time buyer in the area was a young couple in their early 30s with a $37,400 annual income who hoped to get a three-bedroom, detached house for $88,000.

New detached houses meeting that description are difficult to find in the suburbs within reasonable commuting distance from Washington, but O'Sullivan said that a three-level town house with 600 square feet per level would be an answer for such buyers.

O'Sullivan said that while reality often prohibited buyers from finding their dream house at their dream price, such surveys can help builders better target the market.

Dwight Schar, president of NVHomes in Northern Virginia, said that in an effort to keep housing affordable for the baby boom generation, builders may start offering such options in the future as houses that are small but designed so that they can accommodate additions. He said that he also believes the home improvement business will expand significantly, as people find they can add another room for far less money then it would cost to move to a house with one more bedroom.

Overall, many builders agree that housing will have to change to accommodate the newest generation of homeowners.

"People in the future can expect to spend much more than their parents did for housing," said Kevenides. "The old standard of 25 percent of the income going for housing, that just isn't what is happening in this country anymore. People will have to spend 35 percent, 40, 50, even 60 percent."