Washington Gas Light Co. applied yesterday for an 8.5 percent natural gas rate increase in Virginia, where gas bills were raised by 1.9 percent only last month.

If the Virginia State Corporation Commission approves all of the latest request, the average monthly bill for a home heated by gas will increase $3.73, from $39.93 to $43.66, the gas company said.

For customers who use gas only for heating water or cooking, the average bill would go up 67 cents, from $9.93 to $10.60.

In all, Washington Gas Light is seeking an $8.3 million increase in the amount it collects each year from its Virginia customers, who are mainly in the D.C. suburbs.

Natural gas rates in Virginia were increased effective April 5 by amounts that will add $4.6 million a year to Virginia gas bills.

The gas company has a request for a 12.5 percent, $10.9 million a year, increase pending before the District of Columbia Public Service Commission.

A spokesman for the gas company said the bills of many small residential customers in Virginia would not increase under the new rate request because the company wants to change the two-part billing system that went into effect with the April 5 rate increase.

Washington Gas customers are billed a fixed amount called a "systems charge," to cover the gas company's overhead, then pay an additional charge based on the amount of gas used.

The gas company proposed reducing the systems charge from $9.40 a month to $6 for residential customers who use less than 1,000 therms of gas a year. The increase in the usage charge from 27.76 centrs per therm to 31.16 cents per therm would be less than the reduction in the systems charge for small customers.

The $9.40 a month systems charge has proved to be a hardship on small gas users, resulting in their paying more than their fair share, said spokesman Paul Young, the company's manager of news media relations.

The charge has already been reduced in Maryland and the District of Columbia.

The Virginia rate request would also reduce the systems charge for small business users, from $16.20 a month to $6.

Young said the gas company is applying for another rate increase barely two months after the last one because its costs have continued to climb. The April increase was based on the company's cost of providing service in 1976; the latest request would charge customers for costs through 1978.

Also requested is an increase in the rate of return or profit the company is permitted to earn on its investment in facilities serving Virginia customers. Washington Gas wants the rate of return raised from 9 percent to 9.25 percent in Virginia; Maryland and the District of Columbia already permit a 9.25 percent rate of return, Young said.