As the number of Americans who are renting increases, thanks in large part to strong job gains and major Millennial demand, multifamily is seeing healthy absorption and peaking rents. This is great news for the industry, but continues to put a strain on renters' wallets.

In a recent study, RENTCafé specialists looked into the costs associated with upgrading to a bigger apartment or home in the top 25 US cities. The study found the cost for an extra bedroom could be anywhere from $43 to $1,710.

"If you live in NYC, Boston, Chicago, DC or one of the major West Coast markets and want to make the move to a larger apartment, you may need to reach deep into your pockets," RENTCafé creative writer Balazs Szekely tells Bisnow. "Of course this is in part due to the astronomical rents, but the gap tends to be wider in these cities even if you look at it proportionally."

Not surprisingly, New York and San Francisco are the most expensive cities to upgrade apartments in. Renters upgrading from a one- to a two-bedroom are looking at a $1,169 jump in rents in New York, and a $778 jump in San Francisco. To put it in better perspective, for that price renters are gaining an average 190 additional SF in NY, and 243 additional SF in S.F.

"The good news is that these are averages we're talking about, which means there are better deals than the graphs may suggest," Balazs says. "You'll just have to look a bit harder."