The purpose of zoning laws is to protect the value of your property and encourage its best use. Before you purchase the land, check the local zoning ordinance or contact your local planning agency to find out what restrictions could affect you.
Change of Use
In your specific situation, you want to buy a 19th century mansion with a large building behind, which you plan to use as an office for yourself or to rent. Prior to the purchase, check if this change in use is compatible with the local zoning. Zoning regulations prescribe the use to which the building may be put. Your future office might only be permitted in certain zoning districts. You are considering the ownership of a 19th century mansion. In the interest of preserving the residential character of the neighborhood, you might not be able to put a commercial office on your residential property.
Structural additions and improvements to an existing house are regulated by the local building code which works in conjunction with zoning regulations to ensure safety of the structures. You are concerned there may have been additions and improvements to the 19th century mansion you are considering buying, and these features might not have been approved by the required local agencies. If needed permits were not issued, you may be forced to tear down these modifications and make costly repairs to comply with your local codes.
As you consider purchasing this property, it is vital to contact the appropriate local agencies or an attorney to answer your questions and to be sure you are in compliance with all local codes and regulations.