A short report on McMansions links to a National Association of Home Builders survey that suggests building professionals believe the square footage of the average new American house will fall by 2015:

Respondents expect the average, new single-family detached home in 2015 to be about 2,152 square feet, 10 percent smaller than the average size of single-family homes started in the first three quarters of 2010. Overall, 63 percent of respondents expect the average size of new homes in 2015 to be somewhere between 2,000 square feet and 2,399 square feet, 22 percent expect it to be between 2,400 square feet and 2,999 square feet, while 13 percent expect it to only be 1,600 square feet to 1,999 square feet (Figure 2).

Data from the Census Bureau indicates that the average size of single-family homes completed peaked in 2007, at 2,521 square feet, was virtually unchanged in 2008, and then declined in 2009 to 2,438 square feet. Preliminary data for 2010 shows a further decline, down to 2,377 square feet. Although part of the recent drop in average home size may indeed be temporary due to hard economic times, a number of factors lead building professionals to expect home size declines in the long-run: consumers are focused on lowering the cost of heating and cooling their homes; they no longer have sizeable equity in their current homes to finance a much larger one; diminished expectations for house price appreciation has reduced demand for extra square footage in order to achieve appreciation on a larger base; demographics, 29 percent of the US population will be 55+ in the year 2020, demanding smaller homes; and strict mortgage underwriting for the foreseeable future. Combined, these factors will weigh on the consumer to purchase homes based on need more than want.

My interpretation of this is that a majority of builders think new homes in 2015 will be slightly smaller than new homes of today. Additionally, 23% still believe new homes will be larger than 2,400 square feet. Interestingly, there is not reported evidence of whether building professionals think these smaller new homes of the future will be cheaper.

And here is where square footage will be dropped from these future houses:

To save on square footage, the living room is high on the endangered list – 52 percent of builders expect it to be merged with other spaces in the home by 2015 and 30 percent said it will vanish entirely.

“As an overall share of total floor space, 54 percent of builders said the family room is likely to increase,” said Rose Quint, NAHB’s assistant vice president for survey research. “That makes it the only area of the home likely to get bigger.”

In addition, the relative size of the entry foyer and dining room are likely to be diminished by 2015. However, opinions were fairly evenly divided on the fate of the kitchen, master bedroom and bath and mudroom, she said.

The survey methodology is also worth noting – it was sent to a lot of interested parties but the response rate was under 10%:

NAHB’s The New Home in 2015 survey was sent electronically to 3,019 builders, designers, architects, manufacturers, and marketing specialists. The sample was stratified by region of the country (to be proportional to housing starts in each of the four Census regions) and, among builders, by their number of units started.

A total of 238 responses were received, of which 30 percent came from single-family builders, 19 percent from architects, 26 percent from designers, 7 percent from manufacturers, and 18 percent from “other” building industry professionals.

At first glance, this suggests to me that the findings are quite untrustworthy.