The U.S. homeownership rate experienced its biggest drop in 70 years in 2010, declining to 65.1% from 66.2% in 2000, according to data from the Census Bureau. The decline came even as the nation added 15.8 million housing units, increasing the total housing inventory by 13.6%, the Census Bureau said Thursday. Eleven states suffered declines of at least two percentage points in their homeownership rates, led by South Carolina, with a decrease of 2.88 percentage points. Nevada, the state that experienced the biggest housing boom in the nation over the past decade, saw its homeownership rate fall by 2.09 percentage points. The state’s housing units grew by 41.9% from 2000 to 2010. Housing growth outpaced population growth — which was already the fastest in the nation — by almost 7 percentage points. Nevada, also registered the biggest growth in vacancy rates. The state’s vacancy rate, a measure of the share of unoccupied units on the Census survey, rose by 5.1 percentage points in 2010 from 10 years earlier. It stood at 14.3% at the end of 2010. That increase “was almost completely driven by the increase in Clark County,” said Ellen Wilson, a statistician in the Census Bureau’s Economic and Housing Statistics Division, on a conference call Thursday. Clark County, home to Las Vegas, saw a 6.4 percentage point increase in vacant units. Nevada’s vacancy rate was followed by Florida, (up 4.2 percentage points); Michigan (up 4 percentage points); and Georgia (up 3.9 percentage points). The 10 states with the highest housing unit growth rates were in the West and South. After Nevada, Arizona clocked the second-largest gain, as its housing inventory rose by 29.9%, followed by Utah, with a 27.5% gain, and Idaho, with a 26.5% increase. California had the most total housing units in 2010, as it did in 2000, with an inventory of 13.68 million units. Texas was next, with 9.98 million units, followed by Florida, which gained enough housing units to surpass New York. Write to Liz Enochs.