Sears Roebuck & Company has begun notifying customers around the country that the cost of credit is going up because of a change in the way finance charges will be computed.

Beginning in March in this area and on other dates in other states, for customers with outstanding balances owed, finance charges will begin to be accrued from the date of a credit card purchase rather than from the date of billing. The charge is expected to add an extra 15 days of finance charges on the average to the bills of Sears credit customers, according to company officials.

The Sears move, which may be followed by other retail chains, is part of a widespread increase in the cost of consumer credit reflecting the increased costs of money to financial institutions and corporations.

In Washington, where usury ceilings were recently raised, the National Bank of Washington has notified Chextra customers of an increase in rates for the overdraft checking accounts, and officials at Riggs say interest rates on the Riggs Line will also rise. In both cases customers could end up paying the higher interest rates on money borrowed under the old rate.

Interest rates on the Chextra accounts will rise from 11.5 percent to 15 percent effective March 3, according to Kenneth Tercero, senior vice president. Customers who already owe money to Chextra may convert the money owed to an installment loan to retain the 11.5 percent interest rates, or they may retain it if they borrow no more money from Chextra.

"We're giving them the option. We're not just telling them they have to pay 15 percent," said Tercero.

Riggs expects to increase interest rates on the Riggs line from 11.5 percent to 14 percent about April 1, according to Melvin Chrisman, senior vice president and cashier.

"The outstanding balance would have the new rate unless the customer wished to convert to another type of loan or pay it off," he said. Provisions governing overdraft checking accounts, which are revolving credit accounts rather than installment loans, allow for notification and change, he said.

The change in the way Sears computes finance charges will affect only customers who carry an outstanding debt on their accounts, not customers who keep their accounts current, Sears spokesmen said.

"If your billing date is the 15th, and you bought $100 worth of merchandise on the first of the month on the 15th you would bet a bill for $100. If you pay in full, no finance charge appears on the first statement and you pay no more than $100," said Frederick Weimer, assistant general credit manager for Sears.

On the other hand, if a customer doesn't pay the $100 in full, and makes subsequent purchases worth $50 on the first of the following month, the balance would be $100 for 15 days and $150 for the following 15 days. Sears then adds the balances and divides by 30 to come up with the average daily balance for the finance charge.

A spokesman for J.C. Penney said that chain is considering similar changes in its billing practices. "There's a possibility we could go in that direction," he said. "They get a free ride now, and that's awfully expensive these days." rather than stronger. A good bet is that 1980 will yield zero growth an