D.C. may not have any $139 million houses, but we do have more than our share of million-dollar ones. The Office of the Chief Financial Officer released the city’s economic data yesterday for the month of July, and buried among the employment numbers and office-leasing trends is a telling data set measuring the housing market’s boom by the number of units selling for seven figures.

Even at the depth of the recession, in fiscal year 2009, one out of every 10 single-family homes in the District was selling for at least $1 million. That held steady the following year, before rising slightly to 12.9 percent in both 2011 and 2012. Then it started to take off, reaching 17.3 percent in 2013 and 18.6 percent in the 12 months ending with July 2014. And in July itself, the last month recorded, 108 of the 395 single-family houses sold, or 27.3 percent, fetched a price of at least $1 million.

That’s not just a July fluke: Over the three-month period from May through July, the figure was 24.1 percent. Million-dollar condos, on the other hand, remain rare, constituting just 1.7 percent of condo sales in July and 3.3 percent over the past year.

We already know that D.C. has the highest housing costs in the country. But in a city with a pressing need for affordable housing, the seven-figure prices for single-family homes—-many of them rowhouses that just a few years ago would’ve sold for half that—-don’t just squeeze upper-middle-class residents who might have tried to buy these houses. They also keep upping the incentive for developers to buy up once-cheap properties, which may have earlier served as apartments or townhomes for working-class Washingtonians, and rehab them as, well, million-dollar houses, or a few half-million-dollar condos. If the trend continues without redoubled efforts to preserve affordable housing, diverse sections of town could quickly become enclaves of the wealthy.

Chart made using Chartbuilder