Starting gradually next Monday, as soon as meter heads can be refitted, the cost of parking in central Washington on two-hour meters will rise from 50 to 75 cents an hour, payable only in quarters.

The increase was approved by D.C. Transportation Director Thomas M. Downs, who said only four protests had been received after he announced the proposed rise last month.

One protest, from C.F. Stanback Jr., did not contest the higher prices but objected to the "quarters only" requirement. Another, from H.A. El-Amin, objected to the rate increase as discouraging commuter parking and making the city a less attractive place to live and work.

In his official order, Downs said the new rates are "far below the rates charged by the parking garages . . . for short-term parking" and at the same time are "high enough to encourage commuters to park off-street or use public transit, rather than preempting spaces all day by illegally feeding the meters."

About 3,900 two-hour meters will be affected, in an area extending from 17th Street SE, near the RFK Stadium-Armory complex, westward to 37th Street NW, and from the Potomac River northward to Massachusetts Avenue and, in the Dupont Circle area, to Rock Creek Park. The new rates are expected to yield an added $1.1 million a year.

The refitting is expected to take a few weeks.