Converting a single-family home to a multi-family home in NYC

[Chase Patton]

I've been looking into multi-family homes in the area and am finding a lot of single family homes that are set up to be multi-families, however aren't legal ones. The zoning for most of them is R6 (not that I actually know what that means) so I'm assuming that the conversion can be made since it's not R1. I know that an architect will be involved as well as all the necessary changes to plumbing, electric, heating and meters. Does anyone have any experience in this and have an idea of the costs? I'm trying to see if it's worth buying a single-family and converting it.