In a city with apartment buildings known for their dizzying heights and grandeur, walk-up apartment buildings in New York City have long had a lower, less glamorous profile. Walk-ups are buildings that have just five or six stories of apartments in them, and, as their name suggests, they do not have an elevator. Walk-up buildings reflect a style of construction of the early 20th century, and the flights of stairs to climb every day have been a deterrent for New Yorkers looking for an apartment to rent in the city. But that was in the past. With the trend of increasing rents sweeping through rental buildings in Manhattan, the apartments in these small walk-ups represent a gold mine for the landlords of such buildings.