The little-used practice of carving out a rental apartment from an existing house or back yard could help solve the country's housing affordability problem without resorting to government subsidies, according to a new report.

Based on a survey of 47 communities nationwide with zoning laws that permit the installation of "accessory apartments," Washington-based researcher Patrick H. Hare concluded that these self-contained homes could add 48,000 rental units a year to the country's low-income housing stock.

Other housing officials, including Robert J. Sheehan, chief economist for the National Apartment Association, said that they believe that national construction figure is unrealistically high, but added that they do expect the acceptance of the housing product to grow.

"The idea has potential but has to gain more acceptance and I think over time it will," Sheehan said.

Accessory units are known by a variety of names, including accessory cottages, echo housing, granny flats and second units.

The conversion option is considered particularly well-suited for elderly homeowners who do not want to give up homes too large for their current living needs.

Hare said his research leads him to believe that, as the public become more receptive to accessory housing, an installation rate of one accessory unit per 1,000 single-family homes is achievable in most communities.

When applied nationally, that installation rate translates into 48,000 rental homes a year, he said.

Hare estimates that 40 percent of the nation's counties have amended their zoning loans to permit accessory units.

Of those, Hare said, few actually promote the conversion idea among homeowners and many communities have thrown up obstacles that discourage the practice by, for example, charging an expensive building permit fee.

"We have got enough housing in this country, but we just don't allow people to use it," Hare said.

About one-third of single-family homes in the country could accommodate a second living space, Hare said. According to data collected by the federal government, he said, there are 15 million single-family homes of five rooms or more occupied by households of only one or two people.

Renters typically will find that accessory units are more affordable than other rental options, Hare said.

In Montgomery County, for example, the average monthly rent for 108 accessory apartments rented to unrelated tenants was $140 less than the mean rent for similar, conventional apartment units, according to county data.

About half of accessory units, Hare added, are rented to family members at little or no rent.

One reason rents compare so favorably, Hare said, rests with low construction costs that are about two-thirds less than what it costs to build a conventional rental unit.

In the Washington area, an architect-designed addition probably would cost about $32,000 while a homeowner-conceived plan would cost closer to $25,000, he said.

Melvin Tull, Montgomery County's building code enforcement chief, said, "There is just no cheaper way to build low-cost rental housing, and you don't have to have government money to sweeten the deal."

Locally, three Maryland communities had installation rates that put them in the top 10 communities surveyed. Accessory units are going in at an annual rate of 3.6 per 1,000 single-family homes in Havre De Grace, at the north end of the Chesapeake Bay, 2.3 in Frederick and 1.1 in Calvert County.

In Montgomery and Howard Counties, however, the installation rate is closer to one per 10,000 single-family homes, according to the survey.

Fairfax County has shown relatively little activity, with just four accessory units installed last year.

Despite the ostensible advantages of accessory housing, many housing experts wrote off the concept shortly after the idea gained some currency in the early 1980s but failed to catch on in significant numbers, Hare said.

Local political opposition to zoning law changes that would permit accessory apartments constitutes the largest problem preventing their spread, said Katrinka Sloan, a consumer affairs official with the American Association of Retired Persons.

Opponents argue that these second homes would hurt surrounding property values or contribute to excessive noise and congestion in an area.

However, Sloan said, those fears amount to "a lot of fiction about the perceived impact."

Last September, the Federal National Mortgage Association (Fannie Mae), began an experimental program to encourage alternative elderly housing options, including accessory apartments.

To date, lenders have produced a handful of accessory unit loans under the program, said Gerald R. McMurray, the company's vice president for housing initiatives.

Hare said he believes that local resistance to accessory apartments will fade once word gets around that some of the country's more high-income communities, such as Greenwich, Conn., and Boulder, Colo., have successfully integrated some of the highest concentrations of accessory apartments in the country.

Communities that do recognize accessory apartments need to promote the idea more heavily, Hare said.

Greenwich videotaped interviews with three homeowners with accessory apartments as part of its efforts to encourage more conversions, Hare said.

Montgomery County is gearing up to promote the idea with the production of a conversion brochure, explaining such things as how much to set rents at and what to expect in the way of construction costs.

"Accessory housing is bound to catch on, but, as in a lot of things, it is going to take some marketing," Tull said.