Before you consider building over any existing extension you should first get a good idea as the the structural condition of the existing extension.
If there is any cracking or settlement in the existing extension it would be advisable to be extremely cautious before building over the extension. If the extension cannot support it’s own weight without settling or cracking, it is highly unlikely to be able to support an added load without further movement.
If the existing extension appears to be structurally sound with no existing movement or cracking, it still is advisable to open up a number of excavation holes beside the existing foundation to determine their depth and thickness.
Also bear in mind for design purposes, that the new first floor extension may not need to sit directly on the existing extension but can fully or partially span over the existing extension to maximise the potential layout for the first floor extension.