The District of Columbia's new rent control law goes into effect today, making possible rent increases as early as May 1 for thousands of tenants and requiring the filing of new rent control forms by every landlord in the city.

The new measure also authorizes rent subsidies for many low-income tenants. But there is no money to finance them, so City Rent control officials say there is no chance that the subsidy program actually can be started before Oct. 1 if that soon.

For occupants of an estimated 150,000 rental units in Washington, the new law will result in near-automatic rent increases ranging from 2 percent to 10 percent, depending upon the extent of electric, gas or other utility services provided by the landlords. Where no utilities are provided, the increase will be 2 percent; where all utilities are provided, it will be 10 percent.

The new measure, voted by the City Council in November, becomes law following 30 legislative days in which Congress did not exercise power to veto it. It replaces an earlier, somewhat different rent control law that had been in effect since 1975.

Bowles C. Ford, the city's rent administrator, said the provisions of the new law will first affect the landlords, with tenants feeling the results no sooner than May 1.

Ford said every private landlord must file either a registration form containing information about each property, or a form claiming exemption from rent control.

Rent increases may be put into effect May 1 only on properties that are registered by March 31, Ford said. Tenants must be given a 30-day notice of the higher rent. Any landlord who misses the March 31 registration deadline must wait until June 1 to collect higher rents, he said.

Al Williams, deputy rent administrator, said the forms filed under the old rent control law no longer will be recognized because the nature of the exemptions from control have been changed.

Under the new law, any private landlord who owns five or more rental units, whether at one location or on scattered sites, will be subject to rent control for all properties, Williams said. Under the old law, units located on scattered sites were not counted.

There are no exceptions to the requirement that all landlords file either form, Williams stressed.The rule applies, for example, to a Foreign Service officer who owns a home - and nothing else - in Washington but rents it to someone else while stationed overseas.

The forms are available at the D.C. Rental accommodations Office in room 439 of the Munsey Building, 1329 E St. NW, or at the Apartment and Office Building Association, 1511 K St. NW.

The new law provides that a landlord may file a hardship petition in place of the 2-to-10 percent increase if the rental property is earning less than an 8 percent rate of return on investment. Mortgage and interest payments are not included in figuring the 8 percent.

However, if the landlord is paying out more cash than he is collecting in rent, the law permits rent increases to bring earnings just above the breakeven point, Williams said.

Landlords seeking higher rents or tenants complaining of excessive rents or other violations may file petitions with the Rental Accommodations Office. In disputed cases, hearings will be held. Information is available by telephone at 724-5600.