Zoning is the critical issue, and usually it's not just a matter of "paperwork" to get it changed. Whole neighborhoods are typically zoned collectively, so at a minimum you'd probably need a petition signed by about 90% of the residents of the area to even get consideration for a zoning change.
Aside, from that, some professions (doctors, dentists) have special requirements with respect to plumbing, electricity, waste disposal, etc that may be very expensive to implement.
