The D.C. Public Service Commission lifted most restrictions on natural gas use in Washington yesterday, a move that will enable countless home-owners and apartment and office building landlords to switch their heating fuel from increasingly more costly oil to cheaper natural gas.

The commission said the Washington Gas Light Co. could start adding about 3,700 new permanent customers a year in the District, a 2,4 percent annual growth.

In addition, the commission said the gas company could start tapping a vast new supply of natural gas that eventually will be available throughout its District, Maryland, Virginia and West Virginia service area for interruptible sales, gas that goes to customers whose supply can be cut off when there is a gas shortage.

Because there is no imminent natural gas shortage, even new customers who buy gas under the provision that service can be interrupted are virtually assured that they will not be cut off, according to one source close to the D.C. people's counsel, the official consumer representative in District utility cases.

Washington Gas has received 3,000 inquiries about changing residential and commercial heating systems from oil to gas in the last two months, according to WGL spokesman Paul Young. That period roughly coincides with oil price increases dictated by the Organization of Petroleum Exporting Countries and a heightened awareness of the nation's energy crisis.

With the Public Service Commission approval for additional customers and extra gas usage, people who wish to switch their heating fuel now can do so, according to Young.

John T. O'Neill, executive vice president of the Apartment and Office Building Association of Metropolitan Washington, said that without the availability of the extra natural gas supplies, apartment landlords typically would have had to boost rents $25 per month per unit this winter in buildings where oil is used for heating systems.

He said that it will cost about $6,000 to transform an oil furnace to natural gas use in a 20-unit apartment building, but that such an expense will be substantially less than the cost of continuing to use oil.

O'Neill said that 42 cents worth of natural gas currently will supply about the same amount of the energy as a 75-cent a gallon of heating oil and predicted that heating oil will cost $1 a gallon by mid-October.

In short, he said, the commission decision "is going to be a lifesaver for the small apartment and small office owner."

Young said that for a homeowner who already cooks with natural gas, it would cost about $1,000 to $2,000 to hire a mechanic to convert a furnace to burn gas rather than oil.

He said that if Washington Gas Light has to lay gas lines into a house, there might be an additional charge. Young said the company estimates the lines exceeds that figure, the customer is charged the extra amount. If it is less than the two year bill estimate, there is no additional charge.

Because of the shortage of natural gas, Washington Gas has been severely restricted from adding new customer in recent years. From 1972 to early 1978 the company was prohibited from adding any new customers in the District except in the 1968 riot corridors. Last year, Washington Gas Light was permitted to add 10,600 new customers in Washington, Virginia and Maryland.

Under the order approved yesterday by D.C. commissioners Ruth Hankins-Nesbitt and Elizabeth Patterson, Washington Gas Light will be able to supply 620 million cubic feet of natural gas to the District's 3,700 new customers each year.

Washington Gas has proposed increasing its available supply for interruptible customers from 1 billion cubic feet to five billion cubic feet.

The commission tentatively approved that use, but said that the gas company should only tap the 1 billion cubic feet of extra amount for use in the District until utility commissions in Virginia, Maryland and West Virginia give their approval for use of the larger amount

Young said that the company has extra gas available again for several reasons. He said the lifting of price restrictions has encouraged more exploration for natual gas, Algerian liquid natural gas is now being shipped into Cove Point, Md., and Washington Gas Light customers are cutting back their natural gas use by installing more insulation and storm doors and keping their homes cooler in winter and warmer in summer.