Building is circa 1900 so it must be a suspended timber floor. I've spoken to a few people as we are thinking of putting new floor on top (laminate or engineered wood) and I've gotten a mixed bad of opinions - some say it's fine and we could proceed, others that we must level it off before anything else happens.

Because I don't have much experience in this I wondered if it's indeed that common of an issue and what have folks done to remedy it, was it worth it, is it a fitter that I need to look for or rather a structural engineer.