SEATTLE—Accelerated mutifamily rent appreciation in the obvious hot spots, such as New York City and the Bay Area, is hardly news by now. But now rents are rising in smaller markets, with some—including Denver, Kansas City, MO and Portland, OR—seeing annual growth at more than twice the national average, according to the Zillow Group.

Data from Zillow show that the usual suspects—including Boston, Los Angeles and Zillow's hometown of Seattle as well as New York and San Francisco—have continued to see rents increase at a faster pace year-over-year than the national average of 3.3%. Likewise for the Texas boomtowns of Dallas, Houston and Austin. However, markets that saw flat growth or even declines two years ago, such as St. Louis, are now catching up.

Meanwhile, Y-O-Y increases in pricing on for-sale homes has slowed for nine consecutive months, with January representing a 5.4% increase from a year ago. Home prices' 0.2% increase from the previous month was half the rate of the month-over-month gain in rents, according to Zillow. The median rent across the US is now $1,350 per month, and Zillow expects rent growth nationwide to surpass growth in home sales by year's end.

Rents currently take up an average of 30% of renters' monthly income, up from the historical norm of approximately 25%. Depending on one's viewpoint, this represents either a long-term issue for home sales or a boon to apartment landlords.

“Since 2000, rents have grown roughly twice as fast as wages, and you don't have to be an economist to understand why that is hugely problematic,” says Stan Humphries, Zillow's chief economist. “More than one-third of Americans are renters, and today's renters are tomorrow's buyers. For many current renters, buying a home could mean both a lower and more stable monthly payment, but rising and increasingly unaffordable rents make it difficult to save for a down payment on a home.”

The rental market, Humphries continues, “used to be and should remain a stepping-stone to homeownership. But given how widespread rental affordability problems have become, the rental market could be acting more like a barrier to buying. More supply will help ease the crunch, both from new construction and as current renters transition into homeownership, creating more vacancies in existing developments. But neither will happen overnight.”

Among the more than 100 housing experts surveyed in the latest Zillow Home Price Expectations Survey, over half said they expected rental affordability to continue to be a problem for at least two more years.