Home prices in the Seattle metro area gained again in May, but the rate of growth slowed, in line with a national cooling-off of a frenzied market, according to the S&P/Case-Shiller 20-city index released Tuesday.

The average price of existing single-family homes sold in King, Snohomish and Pierce counties was up 1.4 percent from April, a month in which the average price gained 2.3 percent over March. The May home prices represent a 9.3 percent annual gain.

Nationally, home prices gained 1.1 percent over the month in May, the same as they did in April. Over the year, the 20-city index also was up 9.3 percent.

All of the cities in the index, except for Charlotte and Tampa, saw their annual growth rates slow, index officials said.

Save 75% on a Digital Subscription Today

“Home prices rose at their slowest pace since February of last year,” said David Blitzer, who oversees the index.

The 20-city index is back to its summer 2004 levels, but still about 18 percent short of its summer 2006 peak. The Seattle market is still about 13 percent below its summer 2007 peak, according to the index.

Stan Humphries, chief economist of Seattle-based Zillow, said the trend in Case-Shiller numbers was consistent with movement toward a more normal housing market. But the averages mask a lot of variation, with lower-priced homes appreciating more quickly than the most expensive homes.

“Even within local markets, the entire housing stock is not performing the same,” Humphries said. It’s a market where “every layer of the onion we peel off reveals something different.”

Sanjay Bhatt: 206-464-3103 or sbhatt@seattletimes.com On Twitter @sbhatt