Build it and they will come. Great in theory. Harder to pull off.

Another curious puzzle in the purportedly supply-short Southern California housing market has popped up: Why are sales sluggish and inventories rising as local builders construct homes at their fastest pace in a decade?

According to data from MetroStudy, builders completed 3,336 homes for sale in Los Angeles, Orange, Riverside and San Bernardino in the second quarter. That’s up 19 percent in a year and the highest standing inventory since the early days of the economic recovery in 2012’s second quarter.

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Tips for landlords going without rental income This growing supply of new homes in the region, in theory, should be welcome news to house hunters. But the added inventory comes amid an unusual year for existing homes coming to market and depressed overall sales activity.

Inventories for existing homes actually rose during the prime homebuying season. ReportsOnHousing found an average 29,684 existing homes listed for sale in the four counties in the second quarter, as inventory rose by 1,922 homes since the start of the year. In the previous five years, the traditional springtime homebuying rush lowered supply by an average 1,739 homes through June.

A key reason for bloated supply? Buyers balked. CoreLogic reports sales of all residences — new and existing — in the four counties in the April-to-June period were down 4.8 percent vs. the previous year.

The real estate industry took a long time to fully recover from the Great Recession’s ugliness. A prime example was the slow return to aggressive new home construction.

But developers aren’t blameless in their current supply dilemma. In their return to serious homebuilding, they may have misread the market’s thirst for higher-end new homes. Note: The median price of a newly constructed Southern California home sold in June: $581,000 vs. $537,000 for the overall market.

Builders’ desires to grab that premium pricing meant 10,707 homes were under construction in the four-county region in the second quarter, up 6 percent in a year and the fastest homebuilding pace in 10 years. That’s just before the recession officially started.

But Southern California’s builders in 2018 face greater competition from the resale market as well as higher mortgage rates, which trim the buying power of house hunters.

This all translates to builders now controlling a swollen share of homes for sale. Across the four counties, newly constructed residences made up 10.1 percent of all homes — new and existing — available to buy as of June. Two years ago, builders controlled just 6.8 percent of regional supply.

Orange County has seen the biggest swing. Builders had 1,022 homes for sale as of June, up 340 homes or 50 percent in a year. That meant new builds made up 15.7 percent of the options for house hunters. Two years ago? Just 6.5 percent.

In Riverside and San Bernardino counties, 1,450 new homes sat unsold, up 112 homes or 8 percent. Builders hold 10 percent of the Inland Empire supply as of June vs. 8.5 percent in 2016.

And Los Angeles County had a new-home inventory of 864, up 84 homes or 11 percent. Builders had 7.2 percent of the homes available as of June vs. 4.5 percent two years earlier.

The growing inventory of unsold homes might create painful flashbacks of the real estate bubble that burst a decade ago.

Let me calm some nerves by noting 2018’s local construction pace is still 56 percent below the 2004-07 homebuilding levels. And the fattened roster of unsold new homes? Just half of what it was in the bubble period.

Some market watchers blame lethargic homebuying on a lack of affordable homes on the market — existing or new. New homes have historically been the priciest slice of the market.

A dash of consumer psychology may be in play, too. The ultra-tight housing supply in recent years may have motivated house hunters to act quickly. Today’s added choices may be cooling homebuying urges.

But if the year’s real estate malaise is simply a case of oversupply, expect to see builders offering more incentives and homeowners cutting asking prices as homebuying enters its typically slower “off” season.

Remember, those same housing discounts — a coveted response to the high local cost of living — may also discourage builders from aggressive development and dissuade homeowners from selling.

That market response may leave the already frustrated house hunter with limited options to buy.