Liam McMahon Belfast & NI cost calculators
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Extensions

House Extension Cost Calculator

Answer five quick questions and get an indicative cost for your extension, priced for your postcode. Northern Ireland runs well below the UK average, so a national calculator will almost always overstate what you should be paying here.

Rates last reviewed 1 August 2026 · how we work these figures out

What are you building?

The shape of the extension changes the cost per square metre more than most people expect.

How big is it?

Measure the external footprint. If you are not sure yet, estimate: you can come back and change it.

m
m
Ground floor footprint 0 m²

What standard of finish?

This is the single biggest lever on the final number.

Where are you building?

Labour rates vary enormously across the UK. Your postcode sets the regional rate.

The first part is enough. We only use it to pick a regional rate.

If a digger and a dumper cannot reach the work, groundworks are done by hand.

What else is included?

Tick anything that forms part of this project. Leave out anything you already have.

Questions

Extension costs: the questions people actually ask

Construction alone runs roughly £1,700 to £2,300 per square metre in Northern Ireland in 2026. Add professional fees and a contingency and a typical mid-specification rear extension of 20 to 30 square metres lands between about £43,000 and £79,000 all in, which is what this calculator reports. Northern Ireland sits roughly 15 to 20 per cent below the UK average, which is why a national calculator overstates the figure.

The three usual reasons are specification, ground conditions and access. A high-end kitchen, structural glazing or a steel-framed opening can add tens of thousands. Poor ground means deeper foundations. And if a digger cannot get to the rear of the house, the groundworks are done by hand, which is slow and expensive. This calculator lets you set all three, but only a site visit settles it.

Many single-storey rear extensions fall under permitted development in Northern Ireland, but the rules differ from those in England and Wales, and permitted development rights are removed in conservation areas and for listed buildings and flats. Building control approval is separate and is required regardless of whether planning permission is. Always confirm with your local council before you commit.

Yes. Extension work to an existing home is standard-rated, and Northern Ireland builders normally quote domestic customers inclusive of VAT at 20 per cent, so the figures here are VAT-inclusive to match the quotes you will actually receive.

Ten per cent is the working minimum on a new extension, and fifteen is wiser on an older property or a site where the ground is an unknown. The contingency is not padding: it is what stops a drainage diversion or a rotten wall plate turning into a stalled job.