Finding original blueprints/permits on a property

[Zach Ziskin]

I've got a property I'm looking at as a possible rehab, but after doing a walkthrough it appears as if it was either a duplex that was made into a single house, or had major additions done.

Where would I go to research the original plans on the property, or check to see if whatever work or additions were done with the proper permits so I don't purchase a potential legal disaster? Thanks!