The cost of metered parking in downtown Washington and adjacent neighborhoods will rise from 50 cents to 75 cents an hour starting Monday, the D.C. Department of Transportation announced yesterday.

In addition, the cost of two-hour meters throughout the city will rise from 40 cents to 50 cents an hour and the cost of all three-hour meters will rise from either 10 or 20 cents an hour.

The 75-cent hourly rate will be charged downtown, on Capitol Hill and in Georgetown, in a new "high-intensity" zone. The transportation department noted that the new rates still are lower than the $1-an-hour charges recently estabished in downtown Chicago and San Francisco.

The area affected by the new 75-cent rate extends from 17th Street near the D.C. Stadium-Armory complex westward to 37th Street NW in Georgetown, and from the Potomac river north to Massachusetts Avenue, plus the area between Dupont Circle and Rock Creek Park.

The 50-cent rate for one-hour meters in the rest of the city and the 25-cent rate for half-hour meters will not be changed.

The increased rates are expected to yield $1.2 million a year in additonal revenue for the city, bringing the total parking meter income to $6 million a year, according to John M. Brophy, administrator of the transportation department's parking bureau. The new rates were established by a formal rule issued by the department.

Parking meter money now goes into the city's general fund, which pays the cost of the most city operations. For the new fiscal year starting Oct. 1, Mayor Marion Barry has proposed that the money be put into a special fund to help subsidize the construction and operation of the Metro transit system.

Congress passed a law last year that required that the District of Columbia along with the Maryland and Virginia suburbs, provide a "stable and reliable source" of funding for Metro in return for continued federal support of the system. "Our parking meter fund is about as stable as source of revenue as you can find," Brophy said.

Although the meter fees will go up on Monday, it actually will take about three months for workers to change all the 8,940 affected meters and to post the new rates. In the meantime, Brophy said, motorists should pay whatever amount is posted on each meter.

The increase in the rate for the downtown one-hour meters comes 15 months after the last increase. At that time, the price for all one-hour meters throughout the city went up from 40 to 50 cents.

The price of metered parking has rised steadily since the mid-1960s when it was 10 cents an hour throughout the city.

In recent years, the city has sought to keep metered parking at about half the cost of the first hour of parking in commercial garages and lots, which generally costs at least $1.50 downtown now. As a result, it usually is cheaper for motorists to park for a short time in the streets but more expensive to park there all day, since most lots have a maximum rate.

The city government also announced yesterday that it is reinstating fines for overdue children's books from the D.C. Public Library.

The library discontinued the fines for children's books in January 1978 while keeping the five-cents-a-day fine for adult books. The five cent fines for children's books have been reestablished because the number of overdue books has increased, officials said.

The maximum fine for overdue children's books will be $2, while the maximum fine for overdue adult books will rise from $2 to $3. Maximum fines for overdue paperback books and magazines will rise from 25 cents to $1, with the new figure applying to both adult and children's publications.