In a action that seems likely to win approval, a Metro board committee recommended yesterday that the rush-hour bus fare in the District of Columbia be raised by a dime to 50 cents on July 1, the first increase on city routes in seven years.

The recommendation was attached to a sweeping change in the proposed rates to be charged for transit trips using a combination of bus and subway rides. The new plan calls for higher fares than proposed earlier for occasional trips but deeper discounts for daily commuters.

The new proposal also would eliminate differences among fares to be charged for combined bus and rail trips of similar lenghts in Northern Virginia. Commuters who testified at public hearings last fall attacked this as discriminatory.

Combined bus-rail fares would become effective at the time as Metro opens its second subway line, 12 miles long, from National Airport through Rosslyn and downtown Washington to the Stadium-Armory station.

Metro plans to feel large numbers of bus routes into outlying stations, notably the Pentagon and Rosslyn stations in Arlington County.

Under the new proposal, a round trip by a commuter from anywhere in Arlington to downtown Washington would cost $1.65, which is 15 cents more than the present round-trip bus fare in rush hours. Under the old, discarded plan that was proposed last summer, it would have cost $1.80 for North Arlington passengers and $2.10 for most South Arlington passengers to travel downtown and back.

This would have occured because the proposed train fare downtown from the Pentagon station, which will serve South Arlington buses, was higher than the proposed train fare from the Rosslyn station, which will serve most North Arlington routes. The new proposal calls for charging the same train fare downtown from both stations.

Commuters from the Anacostia area of Southeast Washington, who were threatened with a $1.60 round trip to downtown - double the 80 cents it now costs to ride on buses - now would pay $1.35 in rush hours. Under the new plan, the cost of an all-bus round trip would rise to $1 in rush hours, narrowing the gap between and all-bus and a combined bus-rail trip.

In recommending the changed plan, the Metro board's committee on revenue, operations and subsidy allocation said it was responding to public concerns that new fare system would be too high and too complicated.

After a brief discussion, which indicated the new fare plan probably would be approved, the Metro board agreed to hold public hearings on it in mid-March. The places and times will be announced later.

The new city bus fare proposal calls for the first rise for Washingtonians since 1970, when the buses were privately owned by the old D.C. Transit System.

After the public Metro transit authority took over in 1973, it kept the old city rate in force, with the District government choosing to subsidize losses rather than risk a loss of bus patronage and add to the personal expenses of low-income riders.

Suburbanites, whose rush-hour fares have risen sharply since the Metro takeover, have increasingly objected to the growing gap between city and suburban fares. For example, it now costs 70 cents for a suburban trip no longer than a typical 40-cent ride in the city.

Last year, Sen. Lawton M. Chiles (D-Fla.), then chairman of the Senate District Appropriations Subcommittee, pressed city officials to adopt a 10-cent fare increase in rush hours. yesteday's committee recommendation is the first publicly disclosed step in that direction.

Also at yesterday's meeting, the Metro board voted to grant suburban riders a third delay of a more immediate proposed bus fare increase. If approved, it would have raised suburban fares Feb. 28 by a range of 10 to 30 cents. Earlier delays were procedural in nature.

However yesterday's expected approval fell apart when District directors of Metro, led by D.C. City Council Chairman Sterling Tucker, insisted that the suburban fare rise be accompainied by a simultaneous lowering of fares for elderly and handicapped passengers at all times.

This was opposed by suburban directors, and board put off final action two weeks. At present, the elderly and handicapped ride for half fare - in the city and suburds alike - only during nonrush hours.

Metro has charged higher fares during rush hours on suburban buses since September, 1975, and on the downtown subway since it opened last March. The rush - officially called the "peak" - hours are from 6 to 9:30 a.m. and from 3 to 6:30 p.m. weekdays, except holidays.

The new recommendation for combined bus-rail fares would provide for lower fares on the trains in nonrush hours only for trips up to three miles long. At other times, there would be no difference between the fares.

On the trains, the proposed rushhour fares would be 50 cents for the first three miles traveled, and 10 cents a mile thereafter. In offpeak hours, the fares would be 40 cents for the first two miles, and 10 cents a mile thereafter. The same method - but different rates, including lower ones for very short trips - had been adopted earlier.

Under the new plan, the discount fares for regular commuters would be provided by selling reduced-price fare cards that would activate the turnistile-like gates in subway stations and would shown to the driver, and accepted like a pass, on the buses.