Because of massive overbuilding and failure to close old unneeded schools, the District of Columbia school system had 19,000 excess seats in its elementary schools last year and will have a surplus of 28,000 seats by 1980, according to a new report by the General Accounting Office.

The GAO said the overbuilding occurred during the last five years as planned construction continued despite steady declines in enrollment.

In many cases, GAO said, the school board built new schools or additions in particular neighborhoods white ignoring half-empty schools nearby.

The projected 1980 surplus is equivalent to 47 of the city's 129 elementary school buildings, which now average about 600 students each, GAO said.

The agency said the school board should set up a system to plan which schools should be closed, and it recommended that Congress reject the board's request to build another new elementary school next year.

In reply, the school board acknowledged the classroom surplus but said many older school buildings still are inadequate. School Supt. Vincent Reed said the board has closed 18 old buildings during the past year. All of them were replaced either by new buildings or housed special programs, and were not regular neighborhood schools.

Reed said that closing more schools would be difficult because surplus seats are spread unevenly throughout the city and that closing particular schools would cause major disruptions in neighborhoods.

Meanwhile, a D.C. City Council committee, while receiving a copy of the GAO report late last week, has recommended that the school budget increase for fiscal year 1979 be held to 1 per cent rather than the 10 per cent required by the school board and the 2.6 per cent recommended by Mayor Walter E. Washington.

Because of required increases in teacher salaries and court-ordered improvements in special education, the school board contended that even the mayor's proposed $6.3 million increase would lead to some reductions in regular schools programs.

School budget officers said the preliminary budget mark set by the Council's committee on education, recreation, and youth affairs - just $2 millionabove the $232.5 million being spent in fiscal 1978 - probably will require reducing the number of teachers by about 235.

Yesterday, school board President Therman Evans strongly denounced the Council committee's proposal to trim the mayor's budget as a "callous" and "insensitive" move.

The entire Council is scheduled to vote on the budget today. It then must be signed by Mayor Washington and approved by President Carter and both houses of Congress in a cumbersome process expected to require almost a year.

According to school system estimates, enrollment in city public schools is expected to drop next fall by about 3 per cent.

The estimated enrollment this September was 122,500-about 18 per cent below the school system's peak of 149,116 students in the fall of 1969.

Despite the drop. GAO said, the city has carried out the largest school building program in its history, spending more than $300 million in the past decade to provide new classrooms for more than 50,000 students.

During the past five years, the District has opened 33 new schools and additions to existing ones.

Most of the new schools have large, open-space classrooms with air-conditioning and are far more expensive to neat and maintain than the buildings they replaced. Many of the schools also contain "community facilities," wings with space for after-school recreation, education and civic programs, mainly for adults.

School board member Betty Ann Kane said yesterday that the new buildings are an important reason for the board's budget problems.

She said regular school programs also have been squeezed by an increase in required contributions to the teachers' pension fund which increased from $5.8 million in 1975 to $14.6 million in the 1977 budget and will go to $218 million in 1978.

But Kane said the most important factor in the schools' budget problems has been the major increase in salaries during the past decade that accounts for about 88 per cent of all school spending.

The pay levels have been set by Congress and in recent years by the City Council.