A change in the renting and buying patterns of unmarried young persons in the past dozen years has contributed to a sharp decline in rental demand since 1973, according to Citicorp Real Estate Inc., Detroit.

A combination of the Equal Credit Opportunity Act of 1975 -- which made it easier for single persons to buy housing -- with the revival of the condominium market and the strong condo conversion trend of the past two years provided a supply of for-sale units that meets the special needs of the single person, said Philip Kozloff, president of Citicorp Real Estate.

"The inflation-hedge psychology of homeownership has caused many singles and young families to become buyers earlier than they otherwise would have," he added.

The rental market in recent years has been heavily dependent on persons 35 and under and unmarried, a group that has been twice as likely to be renting than married persons the same age and four times as likely as older families, he said.

Between 1970 and 1977, the number of unmarried households under 35 nearly tripled, with an increase of 3.2 million. By comparison, the increase in married households of all ages, starting from a base 25 times as large, was only 5 million, Kozloff said.

Absorption of rental apartments (non-subsidized, unfurnished units absorbed within a year of completion) reached a peak of 510,400 in 1973, Kozloff said. In 1975, the number had declined to 214,200 and by 1976 had reached a low of 115,400. The figure for 1977, the most recent available, was 193,600. CAPTION: Picture, SUBSIDIZED HOUSING--Sponsored by the Archdiocese of Washington, this $4.2 million rental apartment building at 725 24th St. NW will have 123 efficiency units and 17 one-bedroom dwellings for elderly and handicapped persons of low and moderate income. Called St. Mary's Court, the dwellings will have a federally subsized rental program for tenants. Oudens & Knoop designed the building. James G. Davis Construction Corp. is the general contractor. Completion is scheduled in April. Photo by Jack Hayes