One reason local homeownership is so low: Few vacant homes.

U.S. Census Bureau data that tracks if ownership residences are occupied shows Los Angeles and Orange counties averaged just 0.9 percent of these residences as empty in the first nine months of 2017. Only 18 of the 75 largest metro areas had higher occupancy rates.

Homes for owner occupancy are empty for numerous reasons, but preparing for a sale is a key one. (This count is separate from analysis of rental unit vacancies.) The limited number of empty houses is helping to trim the already small supply of Southern California homes to buy, a shortage that frustrates house hunters and helps drive up prices.

In fact, L.A.-O.C.’s home vacancies have averaged 0.8 percent in each of the previous three years. Only three other major markets — San Francisco, Silicon Valley and Akron, Ohio — were consistently under 1 percent vacancy since 2014.

Or look at the trend as a sign of growing housing demand since the recession. Before this current streak began, the last time L.A.-O.C. home vacancies ran under 1 percent was in 2005. The L.A.-O.C. vacancy rate peaked at 1.8 percent — double current levels — in 2010 and 2011.

Inland Empire residences are somewhat more vacant. In Riverside and San Bernardino counties, 1.9 percent of homes were empty in 2017’s first three quarters — 52nd highest vacancy rate among the top metros. Inland Empire vacancy rates averaged 1.9 percent for 2016 and 2 percent for 2015.

But San Diego County is tighter, with just 0.4 percent vacancies in 2017’s first three quarters, fifth-lowest nationally. San Diego had 1.2 percent of its homes empty last year and 1 percent in 2015.

(FYI: L.A.-O.C. rental vacancies averaged 4.2 percent this year; 4.3 percent in San Diego; and 6.2 percent in the Inland empire, by this math!)

Similar home vacancy patterns show up statewide. In the third quarter, 1 percent of California ownership residences were vacant — eighth lowest among the states. It’s quite a flip from what happened after the bubble burst: Statewide vacancy peaked at 3.3 percent as the financial crisis brewed in 2008’s first quarter.

Steep demand for California housing has made empty homes a relative rarity. How rare?

My trusty spreadsheet tells me that since 2013 only two states — Colorado and Minnesota — had lower average home vacancy rates than California.

How much Orange County housing can you buy for $350,000?

SEE: $350,000 new home in Orange County? It’s in the works in Rancho Mission Viejo