We have plenty of empty houses. So why are we building more of them?

The housing market was at the center of the financial crisis, so it’s only natural that it’s closely watched during the economic recovery. And Americans have, encouragingly, been buying more homes, but builders may be getting overexcited. A new analysis from Trulia finds that even while single-family homes stand vacant, we’re constructing more and more.

“Newly released data from the Census Bureau’s American Community Survey (ACS) show that the vacancy rate for single-family homes actually ticked up a bit in 2013,” writes Jed Kolko, chief economist at Trulia. “That’s a big surprise. It suggests even today’s low level of single-family construction might still be too much, too soon.”

The next three charts illustrate what Kolko is saying. This chart shows that the vacancy rate has grown since 2006’s booming housing market, and that it indeed crept upward last year.

Meanwhile, single-family home construction, though near historic lows, is creeping upward.

And either way, people are instead flocking to apartments. The vacancy rates of multi-family homes has fallen off dramatically.

Looking at the top two charts, you might say it’s all logical: vacancies have altogether fallen off a little, and construction is picking up a little. And indeed, improving single-family home construction has been seen as a positive economic indicator. But that uptick in single-family vacancies in 2013 indicates there is a surplus of housing inventory. Meanwhile, the demand has also been relatively greater for multi-family housing.

“Even though single-family construction is low by historical standards, it may still be running ahead of demand,” Kolko writes. “If anything, we built slightly more single-family homes in 2013 than we filled.”

It is true, of course, that these figures will vary widely by metro area. Detroit has a surplus of vacant homes, whereas the market in San Francisco is notoriously tight. But Kolko says vacancies remain a problem even in some of the fastest-growing markets:

“In fact, it turns out that it isn’t only economically struggling areas that have stubbornly high single-family vacancy rates. Many faster growing metros overbuilt during the bubble still have lots of vacant homes,” he writes.