A decision issued yesterday by the D.C. Court of Appeals could permit landlords here to collect money denied them under a 1974 city rent control regulaion that was later ruled illegal.

The appeals court ruled yesterday in the case of a Washington attorney seeking the full rent agreed to in a lease he said was signed before the illegal 1974 regulation took effect.

The amount of rent in the lease for the apartment in the 3200 block of N Street NW was $285 a month, according to attorney Jerome Wagshal, the landlord. The rent regulation law cut back the rent to $219 02, Wagshal said.

In July 1975, the D.C. Court of Appeals ruled the rent control law illegal.

Wagshal said he moved as rapidly as possible to restore the rent to $285 on the grounds that he had not been receiving a fair return on his investment.

In addition, he said, he moved to obtain the difference between the rent under the original lease and the rent for 10 months paid under the illegal law. That mount came to $659.80, Wagshal said.

He said that he brought suit for the money in small claims court but that the suit was dismissed by a judge who contended that the issue was too complex for such a tribunal.

Wagshal took the issue to the D.C. Court of Appeals, which ruled yesterday in his favor.

A major question before the court was decide what adjustments might be required in transactions made under a regulation later found invalid.

The court found merit in arguments raised by both landlord and tenant, but said it would be unreasonable and unjust to permit the tenant to use the invalid regulation as an excuse for not paying the rent originally agreed upon.

It was not immediately clear last night how widely applicable the decision would be or how many tenants might be forced to repay rents that had been reduced under law.