There are a lot of factors that are taken into consideration as to the extent of allowable development. Best advice is to check out your neighbouring properties and the the DCC Development Plan and then have a chat with the planner for your area. I’ve just recieved planning permission for my 2-up 2-down cottage in Irishtown. Here, given original building depth of 7m and a plot depth of up to 18m, the policy is to allow for 7m extension at ground floor and 6.25m at first floor.