Loft conversions

Anyone had a loft conversion done or is a builder that does them? If so how much roughly are they? We're looking to create an extra bedroom so not worried about having an en suite in there. Would need a dormer and velux window or two and the hot water tank would need moving as it's in the middle of the loft. Our house was built in 1990 and it's pretty standard I'd say.

Just looking for rough ideas of cost.