ok depends on what you need doing,
we are about to do the same later on this year, we got building permission and building reg last month,
and have been estimated that the cost will be in the region of 10-15k,
bear in mind most of the finishing bits, i'll be doing so its basically just the floor ,dormer shell and stairs that the builders will be fitting.
hope this helps.
also depends the size of the extension, as a structual engineers report may be needed,
so far we have payed out �1200.00 on plans and fees, prior to the work even starting