The Virginia Real Estate Commission today threatened to bring the first suit ever against a Virginia condominium developer over an interpretation of the state's condominium conversion law.

The board said it would sue CBI Fairmac, which is converting the Radnor Heights garden apartment conplex near Rosslyn if it doesn't give the remaining renters more time to negotiate to buy their units or find a new place to live.

Only four persons remained in the four-building, 115-unit complex today, five days after the development was to have been vacated for renovations. All the tenants have been sent eviction notices.

Commission chairman Charles R. Foxx said that if the developer is not "responsive to the commission's request, the commission would consider filing suit to clarify a section of the state condominium law that has been at the heart of the prolonged battle between the Radnor Heights tenants and the developer.

The commission's action fell short of a victory for the tenants, led by Colette Collier and Don Denmark, who asked the commission to halt the conversion. The commission also deferred action on several other tenant requests, with Foxx explaining, "I hope it won't be necessary."

J. D. Lee, president of CBI Fairmac, could not be reached for comment on the commission's action. Ronald L. Walutes, the firm's lawyer, said it would have to study the commission's statement before making any decisions.

The state condominium law requires that tenants be given 90 days' notice of a planned conversion, which also serves as a notice to vacate. During the first 60 days, the law says, each tenant has "the exclusive right to contract for the purchase of the unit he occupies" if it is not going to be substantially altered.

Legal opinions have varied on whether the section means simply that the developer cannot deal with third parties wanting to buy the tenant's apartment or if it also implies that the tenant must be told what his apartment will cost.

Prices have been set for only one of the four buildings, although notices went out to all tenants on May 29. The tenants contend that they should not have been given notice to vacate until the prices for their units had been established. Without knowing the prices, they argue they cannot reasonably decide whether to buy or move.