Virginia's State Corporation Commission yesterday approved an 8.2 percent rate increase for Washington Gas Light Co.'s 167,000 Northern Virginia customers that will raise the monthly gas bill in a typical home about $4.

The $8 million-a-year increase becomes effective Oct. 13.

At the same time, the commission approved a change in WGL's rate structure that will actually result in lower monthly gas bills for more than 35,000 of the Virginia customers - "small volume users" who use less than 1,000 therms of gas each month.

Bill for these customers will go down about 90 cents a month, according to WGL. The average residential customers uses about 1,300 to 1,400 therms - a measure of heat - a month.

The new increase come on top of a 1.9 percent, $4.8 million increase for WGL's Virginia customers that went into effect in April. The company is also approved a change in WGL's rate for its 150,000 District customers.

In its decision yesterday, the Virginia commission also raised from 9 to 9.25 percent the rate of return that WGL is authorized to earn from rate-payers on its capital investment in plants and equipment.

In April the Virginia commission also approved a change in WFL's rate structure so that customer's bills would be broken down into a "commodity charge" based on the amount of gas used and a fixed "system charge" to cover capital costs for the company's equipment.

Bills have been broken down in this way in Maryland and the District since 1976. The idea behind the new structure is to eliminate the traditional "declining block" billing system under which customers paid less and less each unit of gas used as they used more and more of it - a system criticized as encouraging waste.

Under the new system, it is thought by regulatory theorists, the customer can feel that he will see a direct saying reflected in the "commodity charge" portion of his bill if he uses less gas.

But the theories are being put to the test, and many Virginia customers have been angry when they saw the new $9.40 system charge on their bills.

In yesterday's ruling, the Virginia commission reduced the system charge from $9.40 to $6 a month for the 35,000 residential heating customers who use less than 1,000 therms a month.

The company had proposed this and the commission yesterday accepted the proposal as "more equitable" for the small users. While system charges will be reduced by $3.40 a month, their gas rate will be raised as it is for everyone - leaving them with a net monthly decrease of 90 cents.

About 3,300 heating and nonheating commercial customers will also benefit from the small user discount, WGL said yesterday. Their bills will go down about 20 percent.

The increase is $345,000 less than WGL requested.