Do you have a cut roof? I.e. do you need the loft converted or it is already done? If already cut, the cost for installation and make good (plaster finish) would be approx €3000 per unit (depending on the size and the spec).
thanks for your reply. we built our house a couple of years ago. at the time we couldn't get planning for dormer windows - the bedrooms are now all upstairs and we since have got planning. The roof at the front has no velux windows, there is nothing there, so would be putting in the dormer windows from scratch.