The average price of a single-family home in August in the Standard & Poor’s/Case-Shiller 20-city composite index rose 1.7% from a year ago. Economists surveyed by Bloomberg expected the index to show a gain of 2.1% from a year earlier. Data from the ratings agency’s benchmark home price index showed prices fell in 12 of the 20 largest metropolitan areas in August from the year ago. The 20-city composite index for August fell 0.2% from July and the 10-city composite slid 0.1%. Standard & Poor’s said the 10-city composite index rose 2.6% for the month from the year-earlier August.  Analysts said both indicies are down about 28% from the summer of 2006 and now at levels last seen in late 2003 and early 2004. Home prices in California’s three largest cities slowed with gains of 5.4% in Los Angeles, 6.9% in San Diego, and 7.8% in San Francisco, each of which is down significantly from July, according to Standard & Poor’s. Nationwide prices continue to level off as demand cools following the expiration of the homebuyer tax credit, which pulled tons of sales forward into the first half of this year. “A disappointing report,” said David Blitzer, chairman of the Standard & Poor’s index committee. “Home prices broadly declined in August. Seventeen of the 20 cities and both composites saw a weakening in year-over-year figures, as compared to July, indicating that the housing market continues to bounce along the recent lows. Over the last four months both the 10- and 20-city composites show slowing growth, after sustaining consistent gains since their April 2009 troughs.” Write to Jason Philyaw.