Maybe it was rebuilt. The first film showed the Frankensteins as being very wealthy and maybe Baron inherited it all after his death. He rebuilt it on the same spot that his father did his work

The lab wasn't adjacent to the family's ancestral home either. It was in an old tower in the middle of nowhere in the first two movies. The continuity starts getting muddy with this one.

You're right about the location of the lab in the first two, but in Son the lab actually wasn't totally intact. It had a gaping hole in the roof, and was falling apart in other areas as well. That's how Lugosi was able to push the rock down almost on Rathbone's head! Later in the film when he started to continue the experiments, he had to do a little work in the place to fix it up, I think.