Am in the process of doing same at the moment. It's pretty much the same as applying again for a new mortgage, you need all the documentation as you did the first time. My original mortgage is with PTSB who allow you a max combined mortgage of 85% of the final value of the house.
thanks beetroot, is the cost of the extension added in its entirity to the current valuation to generate the new valuation for the final value of the house?