A bank must base its decisions on granting home mortgage loans on the combined income of a couple applying jointly for the money even if the two applicants are unmarried, the U.S. Court of Appeals here ruled yesterday.

The ruling came in the case of a Washington couple, Jerry W. Markham and Marcia J. Harris, who were turned down on a joint mortgage application because they were not married. They have since married. . .

According to the appeals court opinion, written by U.S. Circuit Judge Luther M. Swygert for himself and two other judges, Markham and Harris began looking for a Capitol Hill home shortly after announcing their engagement in November 1976.

A Washington real estate firm found a house for them and they signed a contract. The couple agreed to allow a local mortgage company to conduct a credit check on them, and the company submitted the loan application to Illinois Federal Service Savings and Loan Association -- the company which would have held the note.

The Markhams and their real estate firm agreed to Feb. 4, 1977, closing date and the Markhams terminated a lease, changed mailing addresses and began utility service at the new house, according to the opinion.

On Feb. 3, however, the Markhams learned from their real estate agent that the loan had been denied because they were not married. They were told, however, that the loan could be resubmitted if it included a marriage certificate.

The Markhams filed suit, charging that the decision violated the Equal Credit Opportunity Act. A lower judge threw out their case, but the appellate panel agreed with them yesterday.

In arguing before the appellate court, Illinois Federal said creditors sould not be forced to ignore the "special legal ties created between two people by the marital bond."

The appellate panel said, however, that it failed "to see the relevance of any special legal ties created by marriage with respect to the legal obligations of joint debtors." It noted that the two would be jointly liable for the debt, even though they were not married.

The court said that since the Equal Opportunity Credit Act stated plainly that creditors cannot discriminate "on the basis of . . . sex or marital satus" it was a clear violation for the firm to refuse to consider the combined incomes of the two applicants.

Illinois Federal contended that a ruling such as that reached by the appellate court would force the company to add the incomes of all people who supply for credit as a group.

The appellate court said, however, that its decision only forced the company to treat unmarried couples who apply for loans in the same manner that it treats married couples. It does not affect applications involving more than two people.