I have just had some extension plans prepared for Building regulations and the architect has specified a timber floor, as the extension is less than 10 square metres. At the new wall end he has shown the joists sitting in wall hangers but where joists will meet my existing property he has shown a 8x2 wall plate and the joists then fitting to this. As this wall plate is going to be below DPC how do I protect against damp?

+1 to asking the architect.  Might be as simple as putting a vertical DPC between wall and wall plate.