The addition of accessory apartments to single-family homes increases the value of those homes without affecting the value of surrounding homes, according to a recently released study.

The survey, prepared by George Washington University graduate student Rita A. Calvan for her master's thesis, studied 21 communities nationwide that allow accessory apartments in single-family homes. In some of the communities, the use of such apartments is restricted to family members and, in others, they may be used as income-producing rental units, Calvan said.

Locally, Montgomery and Fairfax counties have legalized accessory apartments, with some restrictions, in the past two years, and the District allows accessory apartments to be used as income-producing rental units in some zones.

Calvan said she believes most real estate agents and homeowners assume the value of a home with an accessory apartment is higher, but said there is a misconception that the values of neighboring houses will suffer. "There is no evidence that home conversions cause the values of nearby properties either to decline or to increase," she said. "Some 76 percent of the survey participants stated that neighborhood property values have been unaffected by conversions; nearly 20 percent felt unable to answer."

However, Calvan said the respondents to her survey warned that too many conversions in a small area could change the nature of a residential neighborhood and lead to a drop in home values.

"Conversions have to be done with controls -- aesthetics, provisions for parking, limits," Calvan said. "One respondent from Columbia, S.C., said he was concerned that some neighborhoods there with large, older homes could be turned into slums."

Local real estate agents said this week that they concurred with the findings of the study, but also warned that too many conversions in a single neighborhood could make homes there more difficult to sell.

"On Capitol Hill and Dupont Circle accessory apartments , are very popular," said Yolanda M. Mamone, a broker for Shannon & Luchs Co. in Washington. "In an area like Glover Park, they can change the tone of the neighborhood if they are not controlled, or if the renters are not responsible."

Mamone said a town house with an accessory apartment will sell for more than a comparable town house without one because the apartment can be rented for extra income and can be depreciated at tax time.

Even a restricted accessory apartment that cannot be legally rented to non-family members is an asset, Mamone said.

"People like them for the older kids who come back to the nest, or for elderly parents, or even as a guest suite," she said.

Calvan said she sent her survey to 45 communities -- mostly suburban but some urban -- that have had legalized accessory dwellings for several years, and received 21 responses. Most of the surveys were filled out by tax officials in the communities, but local planners and other officials also responded.

According to the responses to Calvan's survey, some homeowners -- but not many -- may install illegal accessory apartments to avoid paying higher property taxes. (Respondents to the survey reported that the average property tax increase was $241, with a reported low of $15 and a high of $700.)

However, Calvan said her findings show that a community can encourage or discourage accessory apartments by strict or liberal tax policies.

In her report, Calvan also cited studies conducted in the United States and Canada on group homes for the mentally retarded that show property values are not diminished in neighborhoods where group homes are located. "If housing shared by population segments which are sometimes seen as 'undesirable' does not diminish nearby property values, it is doubtful that homes shared by more average groups will either," she said.