Not only is the average American home getting cheaper, it's also getting smaller.

While home prices have declined from their 2006 highs during the real estate bubble, builders have started delivering less square footage on new homes. The National Association of Home Builders (NAHB) says the size of new U.S. single-family homes completed in 2009 dropped to a nationwide average of 2,438 square feet, reversing a trend of the past three decades.

New single-family homes were almost 100 square feet smaller in 2009 than they were in 2007, according to recently released U.S. Census Bureau data. What's behind the trend? The need to deliver more affordable homes is one reason, perhaps, but according to NAHB, not the main one.

The builders say one reason homes are getting smaller is homeowners want to keep energy costs in check. This growing energy-efficiency consciousness is one of many trends that the association said was likely to continue.

Despite the tendency towards a smaller footprint, overall energy usage has been growing. One reason could be the spread of air conditioning.

Census Bureau data show that fewer than half of all new single-family homes completed in 1973 had air conditioning while nearly nine-out-of-ten new homes were air conditioned.

Regional differences

Not surprisingly, there are regional differences in those nationwide findings. The proportion of homes with air conditioning ranged from a low of 69 percent in the West to a high of 99 percent in the South. The Northeast and Midwest were at 75 percent and 90 percent, respectively.

Still, even as energy use climbs, so does energy efficiency.

"Residential Energy Consumption Survey," a U.S. Energy Information Administration (EIA) report released in 2005, confirms that while both floor size and overall energy consumption have been trending upwards for decades, energy consumption per square foot has been dropping.

The survey shows that new households were smallest from 1970 to 1979, averaging 1,863 square feet. They steadily increased through 2005, according to the EIA report.

Likewise, overall household energy consumption was lowest from 1980 to 1989, but has been rising ever since. However, even as residences have grown, the amount of energy used per square foot has declined from 51.8 Btu per square foot before 1940 to only 33.4 Btu per square foot in structures built from 2000 to 2005.