Washington Gas Light Co., in another indication that natural gas supplies are improving, yesterday asked permission to add more new customers, including industrial users.

The local gas company asked state regulators for permission to add 8,100 new customers in Maryland and 4,000 new customers in Virginia.

The applications -- filed simultaneously with utility regulators in the two states -- said Washington Gas could supply an additional 2 billion cubic feet of gas a year -- about 2.4 percent of normal current gas use.

In addition, Washington Gas asked permission to provide a billion cubic feet of gas a year to new "interruptable" gas customers -- large industrial users whose gas supply can be cut off when there are shortages.

Washington Gas officials said the company has not taken on new industrial customers since 1972, when a moratorium on all new hookups was declared.

The ban on new connections ended last year when Washington Gas received permission to add 3,300 new customers in Virginia, 4,300 in Maryland and 3,000 in the District of Columbia.

The Maryland and Virginia allocations have been used up, but some capacity still remains in the District, where fewer new homes have been built.

No additional hook-ups in the District have been asked in the latest round, the company said.

Last year's new connections were limited to residential and small commercial customers, with limits on the amount of gas each new customer could burn. There are no restrictions in the current application.

J. K. Hughitt, vice president of Washington Gas, said, "Resumption of an ongoing new business program is now possible due to continued improvement in gas supply, a decrease in the meters served by the company and conservation by our existing customers."

He said the company gains about half a billion cubic feet of gas a year because of reduced consumption by present customers -- mostly due to conservation -- and by attrition -- mostly demolition of buildings with gas heat.

The pipeline companies which supply the firm with gas also have more to sell now, and have ended curtailments which forced Washington Gas to stop taking new customers.

The request to add new industrial customers is in keeping with a recently announced U.S. Department of Energy policy of encouraged businesses to burn gas rather than imported oil.

The new customers will be taken on a first-come, first-served basis, Washington Gas said.

Applications were also filed yester-day to add new customers to two Washington Gas subsidiaries. Frederick Gas Co. Inc., in Frederick, Md., asked to add about 280 customers. Shenandoah Gas Co., in Winchester, asked to add about 500 customer in its territory in Virginia and West Virginia.