Over the next two decades, construction spending in the District of Columbia will grow substantially - with the city's share of metropolitan area building continuing to edge higher, reaching a new peak between 1985 and 1990.

But because of government restrictions on thrift institution branching, the city could experience shortage of $400 million annually of mortgage money in five years and as much as $800 million a year within a decade.

These are the major conclusions of a new report on mortgage credit needs in Washington by Michael Sumichrast, chief economist for the National Association of Home Builders, and Maury Seldin, professor of real estate and urban development at American University.

Their research was sponsored by Perpetual Federal of Washington, the area's largest savings and loan association with assets of $1.055 billion on Oct. 31.

Perpetual and Columbia Federal also based in the District, are seeking to open new branches in the Maryland suburbs but have been thwarted to date by the Federal Home Loan Bank Board's policy against branching across state lines.

Thomas J. Owen, the president of Perpetual, said the new study indicates that, with the strong growth of deposits at area credit unions and subzurban savings and loan associations, the survival of D.C.-based S&L now is in danger.

Ironically, Columbia Federal's space for its proposed new branch in Montgomery Mall has been taken over by a S&L with a headquarters far more distant than downtown D.C. - Baltimore Federal with $928 million of assets, Baltimore S&L's may enter the Maryland suburbs of D.C. at will but Washington associations may not.

Bank Board officials fear they would set a nationwide precedent by permitting D.C.-based S&Ls to branch into the suburbs, and many suburban S&L executives don't want the new competition.

But Owen argued in an interview that the Washington situation is "so unique, action here could not be seen as a precedent . . . no other thrist institutions are limited to a market with 22 square miles of taxable land."

Capitol Hill has reacted by hoping that the Bank Board makes a decision and doesn't force legislative combat.

Although it was prepared at the request of the Washington S&L, the new research by Sumichrast and Seldin will be studied carefully because both men have long records of government consultation and extensive housing research. Among their key findings.

Washington suburban S&L deposits are outstripping inner city deposits by more than 4 to one; savings are growing at a much faster rate in the suburbs than in central cities.

In the last 10 years, some 130 new S&L locations were established in the D.C. suburbs while only 33 new offices opened in the city - the lowest ratio of inner city to suburban openings in the 12 cities.

D.C.'s share of total metropolitan area residential construction will double from 7 to 15 percent in 1980 and increase to 16 percent between 1980 and 1985. This will bring back memories of the 1960s boom in D.C. construction, after which city building declined to a low point in the 1971-1972 period.

The city's share of additions and alterations, reflecting rehabilitation of older houses, will continue to grow. D.C. accounted for about two-thirds of such spending in the 1950s but has since dropped to 25 percent. The forecast is that D.C.'s share will reach up to 40 percent in the 1980s.

According to Sumichrast and Seldin, a conservative estimate puts the growth in the need for mortgage and rehabilitation loans in the city at twice that for available money in the early 1970s.

A decade ago, D.C. accounted for two-thirds of all area savings deposits; today, city S&Ls have 44 percent. Of the 12 cities studied, the District had the slowest S&L deposit growth over the recent decade: 80 percent.

Central city deposit growth at S&L's in the other 11 cities were: Dallas-Ft. Worth, 344 percent; St. Louis, 221 percent; Wilmington, 210 percent; Memphis, 198 percent; Kansas City, 184 percent; San Francisco, 164 percent; Louisville, 161 percent; Evansville, 153 percent; Minneapolies-St. Paul, 151 percent; and Cincinnati, 93 percent.

"The disparity between Washington and other cities in their ability to generate mortgage funds for inner city housing may only be attributed to the policy of the FHLBB . . . the impact of this policy is unique in the Washington area because elsewhere S&Ls may branch within their state up to a radius of 10 miles from their home office,"Sumichrast and Seldin concluded.

At the same time, they noted, the federal government is pushing city S&Ls to provide more mortgage funds for the inner cities. "These programs will not work in the nation's capital, however, unless the serious problem of inadequate savings for mortgages is resolved," the authors added.

And Owen said the suburban S&Ls are under no pressure to invest in D.C. properties. Industry statistics show that virtually all suburban S&L lending is in the suburbs. Ironically, during the period of rapid suburban growth here, city-based S&Ls provided most of the funds under government pressure in the years after World War II.

Now that there is a movement of families and individual home owners back to the cities, suburban S&Ls are generating the resources necessary to finance building and renovations. Nationwide and locally, S&Ls are the largest source of mortgage funds.

Per petual, which has wanted to open a branch at the new Lakeforest mall in Gaithersburg, is more fortunate than most D.C.-based S&Ls because it already had opened four suburban Maryland offices ringing the city before government rules prohibited further interstate branching.

The branch application was filed early this year, and Columbia's application has rested in the Bank Board's regulatory files for nearly three years. A hearing was held on Columbia's petition but Perpetual has had no response. Indeed, officials of the regional bank board in Atlanta earlier this year were debating whether they could even accept Perpetual's documents as a "branch application."