And as certain areas of the city have been rezoned from commercial to residential use, Mr. Visconti said, developers find it feasible -- particularly in lower Manhattan where many older buildings retain unused development rights -- ''to buy buildings and add two, three or whatever the lot will yield in the way of additional floor area.''
The phenomenon has generated considerable controversy, particularly from neighbors of buildings experiencing unanticipated growth spurts and, in some cases, people already living in the buildings.
There are immediate concerns, like the din of construction, fear of falling debris and fire safety; and longer-term issues, like access to light and air, the structural integrity of the old building, the historic character of the community and, in grittier districts, the frustration that ''there goes the neighborhood'' (upscale).
Not to mention sudden loss of that precious commodity: view.