The average price of a single-family home fell 0.7% in September from August, as prices dropped in 18 of the 20 largest metropolitan areas during the month, according to the Standard & Poor’s/Case-Shiller index. For the third quarter, the ratings agency’s benchmark 20-city composite index showed a 1.5% decline from a year earlier and a 2% drop from the prior quarter. “While housing prices are still above their spring 2009 lows, the end of the tax incentives and still active foreclosures appear to be weighing down the market,” according to Standard & Poor’s. The 20-city composite index rose 0.6% in September from a year earlier and the 10-city composite climbed 1.6%. The rates have moderated for four months in a row. For the 20-city index, home prices rose 1.7% in August, after a 3.2% gain in July and a 4.2% increase in June. “Another weak report; weaker than last month,” said David Blitzer, chairman of the Standard & Poor’s index committee. “The national index is down 1.5% from the third quarter of last year and 15 of 20 cities are down over the last 12 months. Other than Tampa, there are no new lows this month but many analysts will argue that a double dip will be confirmed before spring. While some of the bad numbers may reflect the end of the government’s tax incentive for first time homebuyers, there are other problems weighing on the housing market.” National average home prices have climbed 4.9% since reaching a bottom in the first quarter of 2009, but are now at levels last seen in the middle of 2003, according to the S&P/Case-Shiller indices. From the peak recorded in the summer of 2006, home prices included in the 20-city index are down 28.6% while the 10-city composite index is off 28.7%. “The national economy is certainly the number one issue for housing,” Blitzer said. “Additionally, there is a large supply of houses on the market and further, hidden, supply due to delinquent mortgages, pending foreclosures or vacant homes. New construction is running at less than half the pace needed to meet normal demand, so a sustained recovery could be a ways off.” Write to Jason Philyaw.