The New York Times:

For rent and utilities to be considered affordable, they are supposed to take up no more than 30 percent of a household’s income. But that goal is increasingly unattainable for middle-income families as a tightening market pushes up rents ever faster, outrunning modest rises in pay.

The strain is not limited to the usual high-cost cities like New York and San Francisco. An analysis for The New York Times by Zillow, the real estate website, found 90 cities where the median rent — not including utilities — was more than 30 percent of the median gross income.

Seattle is not among the "20 cities where rents are highest relative to median gross income," according to this article. (The article doesn't reveal the other 70 cities that are becoming unaffordable for middle-class families, but I'd bet Seattle made that list.)

The reasons cited are multiple, but among them: tighter post-crash restrictions on mortgages, which have led to more people to renting, which is driving up demand for rental units. "The problem threatens to get worse before it gets better," the article says.