The Energy Department yesterday proposed standards for improving the fuel efficiency of new buildings by an average of 20 percent. The move, if approved by Congress, is designed save the country 216,000 barrels of oil a day by 1990.

The proposed standards would set a limit on the amount of energy, as expresed in BTUs (British thermal units) per square foot a building could consume. The amount would vary according to the type of building, such as office or hospital, and the climate in which it is constructed.

Building designers would be allowed to use any specifications and energy-savig devices they wish so long as the total fuel consumption remains within DOE limits.

John Millhone DTE's director of the office of buildings and community systems, likened the standards for new buildings to the government's gasoline mileage standards for new cars. However, he added that DOE had received no assurance from the administration that it will recommend that Congress make the standards mandatory.

According to the Consumer Energy Council of America, some industry groups, such as architests and contracting engineers, intend to oppose mandatory DOE standards because they are tougher than their own and even those put out by the Department of Housing and Urban Development.

If enforced by law, the standards would affect approximately 65 percent of all new construction, including virtually all residences. The only buildings unaffected would be those not constructed with federal loans or federal insurance. If Congress does nt make the DOE standards mandatory, they would apply only to government construction, although Millhone expects they would serve as guidelines in the private sector.

Only 10 percent of the buildings constructed since 1975 could meet DOE standards now.

The cost of designing an energy-efficient building to meet DOE standards would be increased between 3 and 5 percent for a commercial building. But that cost can be amortized quickly through fuel savings.

As an example, DOE officials said, the redesign of a johnstown, pa., office building, done by AIA Research Corp. for HUD/DOE, cost $71,200. But the first year fuel savings of $65,500 allowed the increased design cost to be absorbed in 1.09 years.

Redesign of homes using passive solar systems, more insulation and other techniques to save energy would cost about 75 cents to $1 per square foot. For the average 1,600-square-foot house, this means increased costs of from $1,200 to $1,600. DOE says the outlay will be paid back in between three and five years.

Buildings currently account for 37 percent of the total fuel consumption in the country. Without the DOE mandatory standards, Maxine Savitz, assistant secretary for solar energy and conservation, estimates, new and existing buildings will require an additional 2.5 million barrels of oil a day by 1990. The proposed standards would mean new buildings could save from 17 to 52 percent annually in energy costs.