If you don't use the center of the building, then you only have room for one "ring" of residential units, whose windows face the exterior of the building. If you cut a donut hole down the center of the building, now you have room for two rings of units. Furthermore, in places like NYC, developers are allowed to "bank" the floorspace that comprised the donut hole and add new construction on top of the building so that overall square footage remains the same.
> Furthermore, in places like NYC, developers are allowed to "bank" the floorspace that comprised the donut hole and add new construction on top of the building so that overall square footage remains the same.
Oh, yah, that's cool. You probably have a lot of systems surrounding the building (and ingress/egress) designed for that total volume.
I think there are limits on both. Floor area ratio (https://en.wikipedia.org/wiki/Floor_area_ratio) is how it is coded into regulations. The lot size and the height restrictions create a bounding box that the building needs to fit within.
Common in NYC to do that to say you have the tallest building by height even if you were only approved to build X floors (your service floors don’t count) — from the couple videos I’ve watched about it over the years (see 432 Park Ave).
