The dwindling housing supply in the United States could be increased by roughly 3 million units over the next 10 years if zoning regulations allowed "underoccupied," one-family houses to be converted into separate living units, a housing expert told local officials this week at a conference on "accessory apartments."

George Sternlieb, an urban policy specialist at Rutgers University, estimated that housing could increase by 5 percent over the next decade if owners of detached houses could divide their homes to create extra apartments or "accessory apartments." He spoke at a conference sponsored by the Metropolitan Washington Council of Governments.

Although the formation of accessory apartments is illegal in many areas, speakers at Tuesday's conference recommended allowing such conversions as a way of providing moderately priced housing in a cramped rental market. The rental revenues from the added units also would relieve new homeowners of heavy mortgage burdens and help elderly homeowners retain their houses, the speakers said.

"Given the litany of problems that we have . . . the gaining of a high level of new housing is not going to happen, and the best resource we have is the housing we do have," said conference speaker Floyd Lapp, assistant planning director of the Tri-State Regional Planning Commission for New York, New Jersey and Connecticut.

A new COG report concurs that the legalization of accessory units may result in decreased homeowner costs and more efficient use of housing space--if such legalization is accompanied by regulatory safeguards.

Using Sternlieb's "5 percent" formula for calculating the rough number of housing units that could be formed through accessory apartments, one could estimate the approximately 500,000 single-family dwellings in the Washington metropolitan area would yield 25,000 additional housing units if such conversions were allowed.

Indications are that accessory apartments already are being formed illegally in many areas of the country, several speakers said. Independent surveys conducted in several East Coast communities found that an average of 10 to 15 percent of all their housing units are illegal apartments.

Eight of the 12 Washington-area local governments that have zoning authority prohibit adding an accessory apartment to any residence in a single-family housing zone.

Only Montgomery County specifically allows such additions, the COG report notes. However, Fairfax County officials are drawing up a proposal to legalize such additions, and the Rockville City Council recently considered--with strong public opposition--allowing the renting of accessory apartments.

The current housing situation in the area is bleak, according to COG statistics. Household-formation rates increased 21 percent over the past decade, four times faster than population growth, while average household size declined from 3.09 in 1970 to 2.67 in 1980.

At a time when skyrocketing housing and financing costs continue to discourage home-buying, the rental stock is also dwindling. Also, the Reagan administration is indicating that it will not sustain a high level of funding for multi-family housing construction.

In light of the poor outlook for newhousing, interest is growing in the better use of existing living space, which University of Southern California researcher William Baer says is poorly used. In a 1978 study, Baer estimated that the country has 15 million rental and owner-occupied housing units with excess space. Based on that conservative figure, he speculated that between 850,000 and 1.7 million new units could be formed by encouraging better use of that space.

Baer defined surplus space as that which exceeds an average of two rooms per person in a unit. Two-thirds of the single-family dwellings in the Washington metropolitan area as of 1977 had two or more than two rooms per occupant, a federal housing survey indicated.

It is not clear how many of those units exceeded the two-room-per-occupant criteria established by Baer, who is dean of UCLA's School of Urban and Regional Planning.

Planners from Weston, Conn., and Babylon, N.Y., told Tuesday's conferees about their successful programs to allow accessory apartments, citing the benefits for established homeowners and renters.

But some Washington area officials are approaching the subject of asscessory apartments gingerly.

"We tend to think of accessory housing in idealistic terms," said Arlington County Board member Ellen Bozman. "I would really like to know from these other communities who the people are who convert their houses and who those are who rent," added Bozman, who also is chairwoman of COG's Human Resources Policy Committee.

Bozman, quoting the 40 percent loss of moderate-income rental housing in Arlington County in the last 10 years, said she wants to hear from county residents about the accessory-dwelling issue. She estimated that 15 percent of the county's single-family homes have only one occupant.

Tri-State Planning Committee member Lapp, in his recommendation for relaxed zoning restraints on accessory apartments, suggested that policy makers examine several potential problems with the conversions.

He urged that they consider mandating miniminum apartment and lot sizes, limiting the age of remodeled structures, restricting potentially disfiguring alterations to house exteriors, requiring owner occupancy of the rented building and requiring off-street parking facilites for renters.