Oct 26 (Reuters) - U.S. single-family home prices rose less than expected in August, posting the smallest monthly gain in more than a year, a closely watched survey said on Tuesday. The S&P CoreLogic Case Shiller composite index of 20 metropolitan areas rose 1.2% on a seasonally adjusted basis after posting a 1.5% increase in July. It was the smallest monthly gain since July 2020 and was below the consensus estimate in a Reuters poll of economists for a gain of 1.5%.The data for August "suggest that the growth in housing prices, while still very strong, may be beginning to decelerate," Craig Lazzara, Managing Director and Global Head of Index Investment Strategy at S&P DJI, said in a statement. Prices in the 20 city index rose 19.7% year over year, down slightly from July"s record yearly increase of 20%. The annual gain was also shy of economists" expectations for a 20.0% rise.