Home values in the Boston metro area fell by 5.6 percent year-over-year in the first quarter to $305,800, according to a report from real estate data firm Zillow Inc.

Home values nationwide also fell in the first quarter at the fastest rate since late 2008, suggesting that a bottom will not be seen until 2012 at the earliest.

Zillow said its nationwide home value index fell 8.2 percent in the first quarter year-over-year.

The number of Boston homeowners underwater, those who owe more on the mortgage than their house is currently worth, amounted to 16.9 percent of single-family homeowners. Nationwide, 28.4 percent of single-family homeowners are underwater, representing a peak since Zillow began calculating the data in 2009.

Foreclosures nationwide also rose, following the moratoriums that had been in place in late 2010. In March, one out of every 1,000 homes nationwide was in foreclosure.

Given all those factors, it is unlikely home values will reach a bottom this year, Zillow said, and the firm pushed its forecast out to 2012.

"Home value declines are currently equal to those we experienced during the darkest days of the housing recession. With accelerating declines during the first quarter, it is unreasonable to expect home values to return to stability by the end of 2011," Zillow chief economist Stan Humphries said in a statement.

Almost all of the 132 markets covered by Zillow saw home value declines. Only Fort Myers in Florida, Champaign-Urbana in Illinois, and Honolulu, Hawaii, managed quarterly increases.