Northwest Natural Co. has requested the Washington Utilities and Transportation Commission’s approval of a rate increase that will average about 1.2 percent for residential customers beginning in November.The new rate would add about 67 cents a month to natural gas costs for the average home, the Portland-based utility said.The increase for the average Washington commercial customer will be about 1.1 percent, or about $3.08 a month, under the rate increase request. The utility, which has 72,000 Southwest Washington customers, expects final approval by the commission Oct. 31. If approved, the new rates will be in effect for the year beginning Nov. 1.The request is based primarily on the utility’s wholesale cost for natural gas, plus some annual adjustments based on agreements with the state utility commission. NW Natural said wholesale prices have increased slightly this year after years of decline. Nationally, the U.S. government forecasts that natural gas users will see an average 13 percent increase in heating costs over last year.