What government office do I ask if can build dwelling?

[Brooke Lange]

Looking at a property to purchase. From google imagery the property next door has a large portion fenced with nothing on it (overgrown). I am interested in purchasing this property and putting a MIL cottage on it for my parents.  (would make my property L shaped) What gov office do I ask if this can be done? (want to make sure I can do that before making the neighbor an offer on it). Thanks!