Federal housing officials are investigating complaints that a Falls Church co-op discriminated against three foreign-born applicants who tried unsuccessfully to buy into the 160-unit complex earlier this year.

The probe by the U.S. Department of Housing and Urban Development follows a finding by the Virginia Real Estate Commission that there is "probable cause to believe" the co-op's membership requirements violate antidiscrimination laws.

Officials of the co-op, the Hillwood Square Mutual Association, have denied the allegations of bias. "The whole thing is ridiculous," says association board president Rosemarie Gore. "Nothing looks discriminatory to me."

A fourth complaint, alleging sex discrimination, was filed last week with the Fairfax County Human Rights Commission by a 32-year-old divorcee who contends her marital status was a major factor in being denied membership at Hillwood.

"They do not want a single woman living there," says Jan Koehler, who initiated the complaint. "The questions they asked during the membership interview were incredible."

Koehler says she was asked why she didn't have custody of her daughter, what the circumstances of her divorce were, and whether she had a boyfriend and would be living with someone.

"I'm 32 years old, I'm not a child. I didn't know who they thought they were talking to," says Koehler. "They've got it all wrong -- this is 1980."

Gore declined to comment on Koehler's complaint, saying Hillwood regulations prohibit her from discussing why applications are denied.

At issue in the HUD investigation is Hillwood's policy of requiring that single applicants, or at least one member of a couple applying for membership, be a U.S. citizen. Association members contend the policy is a matter of financial stability for the complex because aliens tend to be more transient.

The investigation grew out of a complaint filed last May by Celia Espinoze, a legal alien from Peru who together with her husband, also a legal alien, was denied membership.

Virginia real estate officials and Fairfax human rights staff members allege the practice may be discriminatory.

"This is an unusual case," says Richard Kast, fair housing administrator for the state real estate commission. "We feel that there has been a violation [in Espinoza's case], but this is not an overt national orgin discrimination case.Our research, however, indicates that citizenship requirements are a for of national origin discrimination."

An investigation by the commission found no residents who were not U.S. citizens living at the complex, according to Kast, although it did show that people of other national origins reside there.

A HUD spokesman said this week the agency will try to determine whether the alleged problems are part of a more serious pattern of discrimination.

"With [this type of] case, we are not talking about a single incidence where a landlord might turn down one aplicant," the spokesman said. "We are talking about a major policy that would discriminate against every single applicant who was of a certain background."

Both federal and Virginia officials said efforts are being made to mediate an agreement between Espinoza and the association in order to head off court action by HUD. But they said attempts to contact the co-op's officers have been unsuccessful.

As for the sex discrimination complaint, Koehler says she can easily afford the expected monthly payments of $325 on her $16,000-a-year salary as as editorial consultant. She says the payments are only $20 more than her current rent and that she shares joint ownership of two homes in Virginia.

"These people have no reason to turn me down. I had excellent credit rating and character references. I taught for eight years. I am a responsible, single person with a promising career," she says.

Gore says the association expects to review Koehler's application at its next board meeting on Oct.14.

Koehler, however, greets this news with little optimism.

"I don't understand what they're trying to do," she says. "I feel as if I've given my private self to them and they kicked my in the jaw."