Assume you have a document permitting them to stay in the Premises after closing ? 

If the assessor does not show the extra square footage it means it's not permitted space.

Type in search words/phrases such as "City" and "short-term rental permits" or "short-term license" or "vacation rental certificate" or something similar.   

Our homes around Charlotte, NC, have increased in price and rents significantly in recent years (thanks to the many newcomers).

Some of my clients have used a company called Rezio for Architectual, Engineering, and Permitting, they are great to work with and handle the whole process. 

You can charge RUBS if RUBS is specified in your lease agreement or made a part of your month to month lease (which is generally not a good idea) and if RUBS is permitted in your local jurisdiction.

depending on the size of your complex and location you will have to get a permit from you state Department of Natural resources. 

It really is unfair that someone can buy a property and be held accountable for lack of permits on old work.

Annual registrations, point of sale inspections, turning a blind eye to the most awful owner occupied housing while being overly strict to investment properties, courts that continue to drag out the eviction process; excruciatingly slow, laborious, and expensive permit processes; evictions that continue to be granted one extension after the next... these are the difficulties you run into in those cities. 

Maybe they added the extra square footage without permits?