Is a PVC Waste Pipe Legal for 3 Family Brownstone?

I recently purchased a 3 family, 5 story Brownstone in Bed Stuy that has PVC pipe for the waste line.  I am being told by my architect and contractors that it is not to code, and to replace it with cast iron pipe, but found authority to the contrary do not want to incur the expense unless it is necessary.  Does anyone know if PVC can be used for a waste line, and where I can find the rules governing this?  If I do have to replace 50′ of pipe and a trap in a basement, any ideas what that should cost?   Thanks! Brett