Auction buyers will have to pay 10 percent more for their successful bids in Washington and Boston, beginning with the fall season.

C. G. Sloan & Co. and Adam Weschler & Son are following the lead of New York auction houses in adding a 10 percent premium to the selling price of each item sold in their auctions.

William Weschler Sr., vice president of the local firm, said his move was a response to the competition.

Donald Webster, co-owner of Sloan's said, "We couldn't compete with the New York houses any other way. Too many of the good things were going to them."

The new rates will reduce the commission paid by the seller. Under the new arrangement the seller will pay the house only 10 percent (15 percent for goods sold for less than $300). The old commissions charged the seller from 20 to 25 percent. No surcharge (except sales tax) was levied on the buyer.

Peggy Shannon of Sotheby Park Bernet in New York said the idea of reducing the charge to the seller and increasing the price to the buyer originated in Europe in September, 1976.

Elizabeth Shaw of Christie's New York office said the idea came to New York when Christie's opened there in May 1977. Sotheby first said it would not institute the charge, but then followed suit in January 1979.