They want to send a warning to mansion builders.
Mansion builders could also buy older houses for demolition farther south on the avenue.
Suddenly what had been blighted was charged full of possibilities, especially for mansion builders squeezed off Fifth Avenue.
The mansion builders didn't go past 96th, and apartment house construction swept up Fifth Avenue in the 1920s -- but how far up was again the question.
But in 1832, owners on Fifth got permission from the city to extend front yards 15 feet out onto the sidewalks, an amenity that particularly attracted mansion builders.
Nevertheless, the partnership developed a following among families of comfortable means, and by the mid-1880's had projects in hand for the Pratts, Hoaglands, Wheelers and other mansion builders.
In 1893, Cook made the first sales, four house lots on 78th Street, and by 1900 about half the block had been transferred to other mansion builders, like James E. Nichols, head of the wine and spirits importers Austin, Nichols & Company.
In the late 19th century, Madison Avenue had been second only to Fifth as an address for mansion builders, and many of the most expansive houses were clustered nearby, around the ridge of Lenox Hill.
Although other mansion builders had arrived in the section before, the scale of the Vanderbilt houses was such that the stretch between St . Patricks Cathedral and the Plaza Hotel, from 51st to 59th Streets, came to be viewed as their domain.
By this time, its singularity had made it well known as the Cook block and although Cook had not made his intentions public, it appears that he planned to sell off the rest of the block to other mansion builders when the time was ripe.
