The D.C. City Council last night gave unanimous preliminary approval to a new rent control bill that would allow rent increases of up to 10 percent a year in a legislative action that generally drew praise from landlords.

During the five-hour meeting that was characterized by a few sharp exchanges among council members, the body repeatedly rejected attempts to add amendments favored by tenant groups, while landlords won an unexpected victory over how much they could increase rents on vacated apartment units.

The bill would control rent increases in an estimated 112,000 apartment units in the city -- more than half the District's available housing -- until April 1985 if it wins final approval from the council and the mayor and is not rejected by the Congress. All city legislation is subject to congressional review.

The bill would take effect April 30, 1981, when the current law expires.

"This bill is a tremendous improvement over the previous four rent control bills," said John T. O'Neill, the landlord's main lobbyist, after the meeting.

"They [the council] took the position that they had to change the old system to save rental housing in the city," said O'Neill, who has sought what he described as better treatment of owners since the first rent control bill was passed six years ago.

But Perk Perkins, a tenant spokesman, said, "It's hard for tenants to get excited about this bill."

Perkins said the council's decision to allow rent on vacant units to be increased 10 percent when they are rerented was a major setback for tenants.

He said such an increase would price these units out of the reach of low- and moderate-income tenants.

But council member John Ray (D-At-Large) who proposed the increase, argued that landlords need the additional revenue to help pay the upkeep on their buildings.

The council also adopted an amendment proposed by council member Willie Hardy (D-Ward 7) that would allow landlords to pass on to tenants the cost of making any government-mandated repairs.

The council also adopted amendments to make it more difficult to assess triple damages against landlords who charge illegal rents. It already contained language making the award of triple damages by the city's rent control office discretionary rather than mandatory.

The council turned aside an attempt by council member Hilda Mason (Statehood-At-Large) to lower the amount of the annual increase and to decrease the amount of profit allowed a landlord.

The new bill allows owners to realize a 10 percent profit, while the current law sets the amount at 8 percent.

Tenants also lost by a large margin an attempt to keep the Rental Accommodations Office an independent city agency and an attempt to allow tenants to deduct the cost of making some repairs when the landlord refuses.

The new bill also replaced the current nine-member rent control commission composed of three landlords, three tenants and three nonaligned members with two lawyers and an accountant.

The bill also ties future automatic increases that can be imposed by landlords to the area's Consumer Price Index. But the increase would be limited to 10 percent even if the index exceeded that figure.

Landlords endorsed this feature because in the past, the city's rent control office prescribed the amount of the automatic increase and it was usually lower than owners wanted.

With rent control having gained some measure of acceptance from all quarters, council member David A. Clarke, (D-Ward 1) was able to fashion a bill this year that was supported by landlords and tenants, although both sides sought modifications.

Last night's votes seemed to illustrate in public what a majority of city council members have been saying privately -- that they believe rent control has harmed the city's housing market.

But they also knew it has become a necessary evil to their political survival in a city where tenants outnumber homeowners by almost 3 to 1.

The new legislation must be passed before the council adjourns for the year, on Dec. 9, or the bill will die.

The council is scheduled to give its final approval in about two weeks.