Home affordability moved higher last quarter, boosted by the lowest mortgage rates in history, a rise in median income, and slow-to-recover home prices throughout the country.

According to the National Association of Home Builders, the quarterly Home Opportunity Index read 75.9 in 2011’s fourth quarter. More than 3 in 4 homes sold between October-December 2011, in other words, were affordable to households earning the national median income of $64,200.

Never in recorded history have U.S. homes been as affordable on a national level. Even on a regional and local level, affordability soared.

Affordability was highest in the Midwest; 7 of the 10 most affordable markets nationwide were in the nation’s heartland.

The Top 5 most affordable U.S. cities in Q4 2011 were:

1. Kokomo, IN (99.2% home affordability)
2. Fairbanks, AK (97.5% home affordability)
3. Cumberland, WV (96.9% home affordability)
4. Lima, OH (96.0% home affordability)
5. Rockford, IL (95.5% home affordability)

These are each considered “small markets”. The most affordable “major market” was the Youngstown, Ohio area, where 95.1% of homes sold were affordable to households earning the area’s local median income.

Not surprisingly, America’s “least affordable cities” were regionally-concentrated, too, with 7 of the 10 least affordable markets located in either California or Texas.

San Francisco (#3), Santa Ana (#4), and Los Angeles (#5) led for the Golden State but, for the 15th consecutive quarter, the New York metropolitan area took “Least Affordable Market” honors.

Just 29 percent of homes in and around New York City were affordable to households earning the area’s median income last quarter. It’s a large jump from the quarter prior during which 23 percent of homes were affordable.

The rankings for all 225 metro areas are available for download on the NAHB website.