The Reagan administration has decided to try to slash the District of Columbia's capital improvements budget by $75 million next year, a move that could delay or eliminate dozens of planned projects such as repairing school buildings, repaving roads, upgrading fire departments equipment and rehabilitating some public housing projects.

According to high-ranking officials with the Office of Management and Budget, the Reagan administration tomorrow will propose cutting the total amount of capital improvement loans that the city can receive for fiscal year 1982, starting Oct. 1, from the $220 million recommended by former president Carter to $145 million.

Mayor Marion Barry's proposed 1982 budget counts on getting $161.2 million in federal loans to finance a variety of projects, including repairing libraries, replacing and upgrading the air-quality equipment at the city's solid waste center.

If Reagan wins congressional approval for his plan to limit the city to $145 million in capital improvement loans, D.C. officials would have to trim more than $16 million from the amount they planned to spend for the various projects.

Since the Reagan proposals still have not been officially announced, city officials declined to speculate on which of the dozens of projects would suffer because of the cutbacks. One or more projects could be eliminated altogether, or city officials could try to proportionately reduce the cost of all of the projects to account for the lost funds.

"If they're going to cut it across the board and let us pick the specific projects to cut," said Alan F. Grip, Mayor Marion Barry's press secretary, "then we're just going to have to sit down and look at what our capital improvements priorities are."

Among the projects proposed in Barry's 1982 budget are $11.3 million for renovating D.C. General Hospital, including replacing obsolete piping and creating an ambulatory and critical-care facility; $2.2 million for renovations at three police precinct station houses; a planned $12.1 million renovation of the District Building, including updating the heating and cooling systems and $474,000 to repair the aging Whitehurst Freeway in Georgetown.

Also threatened by the funds reduction is an additional $129 million in federal matching funds. The city receives that money from a variety of federal agencies and programs usually based on the amount of money the city can borrow beforehand. If Reagan succeeds in setting a ceiling set on how much the city can borrow, the amount of matching funds for which it is eligible could also be reduced.

The Carter administration also proposed a $133 million capital improvements borrowing authority for the city for the current fiscal year, which ends Sept. 30. Reagan's plan would not change that amount.