The total number single family homes in the residential building stock in the United States will shrink by 4 billion square feet in nine years , a new study by a subsidiary of Navigant Research forecast last week.

Over the last 60 years, the number of single-family homes in the United States has grown and the size of each home has increased, growing by about 40 percent between 1975 and 2010. However, the number of single-family homes is now decreasing. By 2021,the total area of single-family homes in the United States will have shrunk by nearly 4 billion square feet (SF), contracting at a negative compound annual growth rate (CAGR) of 0.2 percent, according to a new report using U.S. Census data from Pike Research, a part of Navigant’s Energy Practice.

“For the first time since World War II, the United States is experiencing increased levels of urbanization,” says senior research analyst Eric Bloom. “As more people move into cities, they tend to occupy apartments, condominiums, and other attached multi-unit housing types. By 2021, over one-fourth of the residential stock of the United States will be in multi-unit residential buildings.”

Overall, according to the report, the U.S. residential building stock will grow from 264.3 billion SF in 2011 to 280.1 billion SF in 2021, expanding at a CAGR of 0.6 percent. That growth will be dwarfed by the expansion in residential building stock in some countries in Asia Pacific – particularly China, where residential buildings will grow by 60 percent in the next decade, reaching more than 600 billion SF by 2021. Most of this new space will be in attached residences, especially apartments, in China’s dense urban centers.

The report, “Global Building Stock Database”, provides data and forecasts on the size and growth of the global building stock from 2011 to 2021 as well as a qualitative description of key drivers and trends in the building stock. The data covers eight commercial building types (office, retail, education, healthcare, hotels and restaurants, institutional/assembly, warehouse, and transport) and two residential building types single-family detached and multi-unit residential) for seven regions worldwide.

According to the National Association of Home Builders, the housing boom drove new to record levels in the middle of last decade, peaking in January 2006 at a rate of nearly 2.3 million single family homes. But then the bubble burst in late 2006 and 2007 and construction ceased in most parts of the country. Starts plunged to just 478,000 homes in April 2009, the low point during the housing bust.

Construction of new homes slowed slightly in July to an annual rate of 746,000, down 1.1 percent from the revised June rate of 754,000, which was a seven-year high. The decline was concentrated in the single-family sector where starts fell 6.5 percent to an annual rate of 502,000, again down from an elevated rate of 537,000 in June, which was the highest since the end of the home buyer tax credit in 2010, NAHB reported.

While single-family starts were down in July, multifamily construction continues to expand. Housing starts of units in buildings with five or more apartments came in at 229,000 seasonally adjusted annual rate, up 9.6 percent from the revised figure for June. The three-month moving average has been very stable, hovering between 205,000 and 210,000 for the past quarter. On a year-over-year basis, housing starts for 5+ units are up strongly, 30 percent since July of 2011.