There is no simple answer to this. How as this come about? Is the window an after thought ie since the house was built. What are the structural details ie brick/block wall, timber frame etc? In the worst case senario the window will have to be removed, the wall supported using needles and a lintel inserted. It is unlikely that the opening size will remain the same and a new window will have to be fitted. Are we talking single, one and a half or two storey. Is it possible that the surveyor is wrong?