This must be the most debated quandary outside of property prices in Ireland.
The answer is simple. What are you building?

What does a car cost? Depends on what type of car, is it new or second hand? Is it a people carrier or a German luxury sports car? Has it leather seats, has it 2 doors or 4?

It is the very same for building. Almost every house is different, some have double glazed PVC windows, some have quadruple glazed alu-clad. Some have red deal floor boards some have American White Oak wide plank solid flooring. Some have fibre cement slates on the roof, some have roofs made of copper or zinc.

An ultra standard house could conceivable be build for approximately €70 per ft2 plus vat but this is a simple design (rectangular, plaster finishes, fibre cent slate roof, double glaze PVC windows) with no finishes (tiling, flooring, decoration, patios).

There are many examples of this type of house scattered in housing estates throughout the country.

What drives up cost per square foot?
As a general rule, the more complicate the design, the more expensive to build
Site conditions, eg, is the site on a slope? Is it well drained?, Is there rock in the ground?
What level of finish is proposed?

Extensions are a higher cost per square foot and much more difficult to pin down. Certainly well over €100 plus vat. There are many factors driving up this cost. Restricted access, demolition and propping, slow works to connect between extension and the existing house, drain and service diversion for example.

Even builders vary in how they cost extensions and houses. Sometimes they get it right, sometimes they don’t. If they can’t get it right, what chance have the rest of us in simplifying it to a mere cost per ft2?

Remember also, a building does not cost the original estimate quotation, but the final account.

If a quote looks to good to be true, it probably is, you get what you pay for.