Extension costs in Ireland depend on size, structural complexity, how much of the existing house is altered, and the level of finish. Extensions often cost more per square metre than new-build because of the work involved in opening up existing structure, matching services and working around an occupied house.
The construction cost of an extension is driven by floor area, but the works that connect it to the existing house often dominate the budget. Removing load-bearing walls, forming new openings, upgrading the existing heating system, relocating drainage and making good disturbed finishes can all cost as much as the new structure itself.
Building regulations apply to extensions even when planning permission is not required, so allow for design, structural certification and, where relevant, building control notifications and certification fees.