Virginia's State Corporation Commission approved yesterday a $4.8 million rate increase for customers of Washington Gas Light Co., to take effect April 5.

The agency's decision was announced shortly after the company's annual meeting of stockholder and was denounced immediately by the gas company's president, Donald J. Heim.

For WGL customers throughout Northern Virginia, who account for 29 percent of the utilitp's annual revenues, the decision yesterday means an estimated 1.9 percent increase in average monthly bills. Exact amounts of the new rates will not be computed until today or tomorrow, a company spokesman said.

In addition, the Virginia agency approved a significant change in the method by which Washington Gas computes its bills. The SCC said Washington Gas in the future may charge a basic monthly charge plus a charge for actual gas consumption.

This new rate structure, already in effect in Maryland and the District, is designed to encourage conservation of gas by charging all customers a flat rate per unit of natural gas consumed; in the past, discounts had been offered for larger volume use.

However, the new rate structure has been the subject of much misunderstanding by Maryland and D.C. customers, whose bills now include two charges (basic systems charge plus gas used. The overall bill for residences is about what it would have been under previous rate structures, however.

In the Virginia rate case, first filed in May 1976, Washington Gas sought an increase of $8.6 million based on 1976 costs. The SCC approved a 5 percent surcharge in October 1976 and the agency said the $4.8 million approved yesterday includes the amount of the surcharge.

Heim said the SCC action was "highly disappointing . . . based on inflation since [1976] the amount falls far short of our present requirements." The SCC said the higher rates will permit WGL to earn 9 percent from its business in that state compared with less than 7 percent in 1976.

But the statement by Heim and earlier comments by Washington Gas Chairman Paul E. Reichardt, at the stockholders' meeting, indicated that the Washington-based firm may seek another Virginia rate increase in the near future.

Last December, Washington Gas received a rate increase of $2.8 million in Maryland (out of $3.2 million sought). In the District, hearings are in progress before the public service commission on a $10.9 million annual rate boost. Reichardt said his firm "may file" requests for higher rates this year in any jurisdiction where its profitability is below permitted levels.

In an address to stockholders, and in responses to their questions, Reichardt generally was bullish about the fortunes of Washington Gas and the natural gas industry.