The average cost of building an extension varies a lot. As a guide an extension with a good standard of construction and finish to todays standards and in compliance with the building regulations would cost between €900 and €1200 per square meter squared excluding VAT in Dublin in 2015. Prices tend to be 20 to 30% cheaper outside of Dublin due to greater availability of labour and easier serviceability. Valued Added Tax at 13.5% is charged on all building works.
As an example a standard rear 30 square meters rear extension (see Extension Picture on the left below) with apex roof, two SO6 velux windows, large UPVC french doors, two UPVC windows, insulated to SEAI U-value standard and painted finish would cost around €27,000 to €35,000 ex VAT and the build time typically 8 to 10 weeks. A more high specification construction 40 square meters with a box design (see Extension Picture on the right below) would cost €60,000 – €90,000 ex VAT.
When VAT is included and the specification of kitchen/tiling/heating/windows/sliders and furnishing and fixtures are included the cost can run to ranges between €50,000 and €120,000. Your architect can give you a good idea of costs.
The cost of an extension depends on where you live, what kind of structure you decide to build, how complicated unavoidable matters such as drainage are and how large it is going to be. You can expect to spend around 10 – 15% of the construction cost on fees for architects, structural engineers, building regulations checks and planning permission (should you need it). There also might be extra costs involved if you need to manage without a kitchen or rent accomodation elsewhere while the work is done.