I am concerned about the crack that has been growing where an addition was built onto the house many years ago. Just looking for some possible reasons for it and advice as to who I should get in to take a look at it.
I would suggest this is something that needs to be assessed by a qualified structural engineer. I'd hazard a guess that the addition has subsided because an insufficient size or quantity of piers has been used in the construction, or significant ground subsidence has occurred over time. However, providing a reason behind the issue is best left to the professionals.