Be v careful. Adding to a raft changes centres of gravity of loads, which changes bearing pressures across the footprint; in turn, that can lead to movement, depending on what's underneath. As the extension is effectively three-sided, the natural tendency will be for the new raft to impose greater bearing pressures under itself to the side away from and parallel with the existing building, unless the connection is designed by looking at the existing and new raft working as one.
The alternative is to design the new raft so both its and the load CofGs are in the same place, so that the bearing pressure is equal, usually by extending it out beyond the outer parallel wall. Only nominal fixings, such as steel dowels resin-bonded into the existing raft at, say 600 centres, would then be required; or it can be left entirely free from the existing one. This requires some knowledge of the ground conditions.
Why does the BCO suspect it's on a raft? If the building's not that old, they should have the records/design/details for what was actually done. Are you sure that you actually need a raft for the extension?
Sounds like you need an SE to take a look at it for you.
