For ya plasterers, sparks etc go to the boozer and ask...pay cash in hand

20k imo may be a little ambitious, but could be about right.

Just to make you a bit more pissed with the price, recently i priced our loft conversion up as we are considering it, and it came to 4k. Similar to yours, but full length dormer one side and velux on the other....and that included converting the small bedroom into a en suite to the master