I just bought a house and I have a problem with my garage ing up against my neighbors garage.?

I bought a house in CA and the home inspector couldn't inspect the garage cause it was filled with stuff. When I got it cleaned out I found that a large section of the wall had been cut horizontal opening up the back of the garage. Apparently, the previous owner made a makeshift wall and extended the roof so that it is right up against the neighbors garage. I noticed that the roof job was clearly visible from the backyard without entering the garage (something I missed earlier). I want to repair this but the cost and effort may be substantial. Is there any rights I have against the inspector or previous owner?