The present current decline in the number of rental housing units will accelerate throught the 1980's, resulting in higher rents and lost mobility for renters, a top housing economist predicts.

Michael Sumichrist, chief economist of the National Assn. of Home Builders (NAHB), writing in the Mortgage Guaranty Insurance Corp. newsletter, attributes the expected decline of the rental sector in the 1980s to "shifting population age distribution which favors home ownership instead of renting."

He added that trends such as condominium conversions and co-operatives, plus a long list of economic and market problems and regulatory changes, contribute to the reduced availability of rental units.

"Something has happened to the once-valid reasons for wanting to rent rather than to own." responded Edward E. Edwards, editor of the GIC newsletter. "The majority of American families cannot affort to rent, not because of insufficient income but rather because home ownership is so much cheaper."

Sumichrist said the country "is losing as much as 2 percent of its rental stock annually" and pointed to a loss of nearly 250,000 rental units a year between 1973 and 1976. He also said the rental vacancy rate at the end of the first quarter of 1979 was "a meager 4.8 percent nationally -- the lowest on record since World War II."

In the long run, Sumichrist said it is "inevitable" that politicians will turn from "a healthy investment climate that would encourage apartment construction to rent controls and restrictions on conversions."

He opposes such a maneuver, saying that it will further discourage apartment construction and "lead to a rash of abandonments by building owners who can no longer make a go of it."