It's not unusual to find uneven/ dipping floors in an older property. This can be due to a number of factors including: inadequate structural supports, undersized timbers, overloaded floors, rot or decay, removal of intermediate walls etc. Structural reports (such as those as generally carried out in connection with a potential purchase) are limited in that they only identify readily visible defects which are not covered up. In order to rule out rot or decay you need to carry out some opening up works, which of course are destructive by nature. A surveyor with experinece in fdealing with older proprties should be able to advise you (with the aid of a moisture meter readings) as to the likelihood of wet or dry rot. Woodworm infestation is generally easily identified. Ask your surveyor for his views, with the knowledge that no restoration building project can be accurately costed without opening up of all covered areas.