Rents have jumped 20% nationally over the past five years, thanks to Millennials' suburban exodus—but some of the biggest multifamily markets might see that slow after an unexpected supply spike.

Many of the nation's biggest cities can expect increases in the amount of new multifamily product through 2016 and 2017. In 25 of the largest US cities, multifamily permits in urban areas were up 39% in 2015 compared to 2014, reports a study by housing research firm Zelman & Associates.

The Zelman study projects next year's multifamily supply growth in NYC and Boston to be around 2.6 and 2.5 times the cities' respective historical averages, the Wall Street Journal reports. Zelman projects Philadelphia's growth to be twice the city's historical average.

Even real estate legends are feeling the excess supply. Sam Zell’s Equity Residential lowered growth expectations this year—from between 4.5% and 5% to between 4% and 4.5%—due to an expected slowdown in the NYC and S.F. markets. [WSJ]