Thanks for all the advice so far

Here is a prelimanary plan of the new extension (left of existing
house), dimensions (red lines) are approximate:
http://img73.imageshack.us/img73/1793/205gh.jpg

There is a 4" soil drain which will go under the rear wall of the
extension. Do I run the footings to either side of the pipe leaving
approx 6" gap and then the brickies will place a lintel to bridge the
pipe?