CNBC's Mark Fahay sifts through Zillow data that show that the median price of a one-bedroom apartment leapfrogged the price of a two-bedroom late last year, and the gap continues to grow.

The rate of appreciation for two-bedroom apartments is about 3%, while the median rent on a one-bedroom apartment has grown around 5% per year. But the numbers are skewed by a certain number of high-cost metros, such as New York City and San Francisco.

"Hip, new multifamily buildings are being built, and those are more likely to be one-bedroom homes," said Skylar Olsen, senior economist at Zillow. "Across the whole housing stock, it could be that one-bedrooms are more expensive because they're in more attractive parts of town."

Rents are also increasing faster in places that have more one-bedrooms already — like New York or San Francisco. Skyrocketing rents in those places lead to increases in the median cost of one-bedrooms nationwide.

"In the middle of the country, land is pretty cheap and space is pretty cheap, so you're more likely to have housing stock with two-bedrooms," said Olsen. "In places where land is more expensive, like downtown New York, that's where rent is exploding and appreciating faster in the tight urban areas."

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