Buyers who wish to purchase a home outnumber the available housing stock by more than 3 million, according to a recent report by Freddie Mac. However, the situation varies considerably throughout the nation, with 21 states having a surplus of properties available for sale.

Freddie Mac previously estimated that 2.5 million additional homes would be needed to make up the shortage in housing inventory in the United States. In a recent Insight report, it says that this deficit increases to 3.3 million when only considering the 29 states where housing demand exceeds supply.

The report says the deficit has been created by several years of underbuilding, especially in states with robust economies that have attracted a larger population. The severe shortage of homes in several of these states—including California, Florida, Oregon, and Texas—has caused some migration to more affordable states, which in turn pressures the housing inventory there.

"We are in the midst of a demographic tailwind, and we expect home purchase demand will remain strong well into the next decade as the peak cohorts of millennials turn 30 years of age in 2020 and beyond," said Sam Khater, chief economist at Freddie Mac. "Simply put, new housing supply is not keeping up with rising demand. We estimate that the housing market is undersupplied by about 3.3 million units, and the shortage is rising by about 300,000 units a year."

The report used data from the U.S. Census Bureau to estimate the number of housing units in each state as well as the vacancy rate, comparing the rate in 2018 with the state's historical rate between 1970 and 2000. Freddie Mac says too high a vacancy rate indicates a stagnant market, while too low a rate indicates that demand is outpacing supply.

The supply crunch was most evident in Oregon, which had a 2018 vacancy rate 6.5 points below historical norms. The vacancy rate was 4.02 points lower than the historical average in California, 3.79 points lower in Minnesota, 3.43 points lower in Florida, and 3.14 points lower in Texas.

Other states showed a more pronounced vacancy rate due to shrinking populations. The 2018 rate was 6.72 points above the historic norm in West Virginia, 6.32 points higher in Arkansas, and 4.69 points higher in Alabama and North Dakota.

In Connecticut, the 2018 vacancy rate was 1.27 points lower than the historic rate. It was 0.37 points higher than the historic average in Rhode Island.

Freddie Mac's static estimate of the housing deficit suggests that Oregon's housing supply is 8.8 percent short of demand, while the shortfall stood at 5.74 percent in California, 5.37 percent in Minnesota, and 5.13 percent in Florida. West Virginia was overstocked by 7.12 percent, while the housing supply exceeded demand by 6.23 percent in Arkansas and 4.45 percent in Alabama.

Both Connecticut and Rhode Island showed a smaller housing deficit. In Connecticut, Freddie Mac found that the housing supply was 2.49 percent below demand, while in Rhode Island it was 1.09 percent lower.

However, the report also took interstate migration into account, noting how factors such as economic opportunity, housing costs, and rents have attracted people to some markets and driven them away from others. When this was taken into consideration, Connecticut's housing deficit fell to 2.01 percent while Rhode Island's dropped to 0.06 percent.

The report suggests that the higher housing costs caused in part by constrained supply have limited household formation, particularly among younger households. Freddie Mac estimates that 400,000 more households headed by 25-to-34-year-olds would have formed but for higher housing costs.

Freddie Mac noted how the housing shortage has led several states to loosen zoning restrictions to address the issue. This response has often involved the elimination of exclusive single-family zoning, allowing developers to build triplexes or other higher density housing options.

The National Association of Realtors has also frequently warned that a shortage of housing inventory is posing affordability challenges, as the median price for a home sold in the United States has risen year-over-year for nearly eight years. In its latest existing home sales report, the National Association of Realtors said the total housing inventory in the U.S. in January was at its lowest point since 1999.