About Liam McMahon

Liam McMahon runs Liam McMahons, based at 2 Killane Road in Ahoghill, County Antrim, and is the creator of the calculators on this site. He also runs Liam McMahons, the independent clothing shop trading under the same family name. They cover extensions, loft conversions, insulation, roofing, driveways and building materials — every one of them priced for Northern Ireland, using the numbers that come across a merchant's counter every day rather than figures lifted from a UK-wide average.

Built for Northern Ireland prices, not UK averages

Nearly every building cost calculator online quotes a UK average. For someone planning an extension in Belfast, Lisburn or Ballymena that is not a small inaccuracy. Northern Ireland consistently runs about fifteen to twenty per cent below the UK average, and a national tool will hand you a budget several thousand pounds too high before you have spoken to anybody.

That is the reason this site exists. Every calculator here starts from a UK-average base rate and then applies the multiplier for the postcode you type in. Enter BT9 and you get Belfast prices. Enter SW1 and you get London prices. The method is published in full on how we calculate, including every multiplier, so you can argue with it if you think it is wrong.

What we are trying to do

Give homeowners in Northern Ireland enough information to tell a fair quote from a bad one. Not to replace a builder, and definitely not to replace a site visit, but to close the gap between having no idea and holding three quotes that vary by twenty thousand pounds with no way to judge them.

The insulation tools

The insulation calculators go further than the rest. They model U-values before and after using the same conventions as a SAP or RdSAP energy assessment, and they are explicit that an improvement to one wall or one ceiling is not a cut to your whole heating bill. There is a lot of loose arithmetic published about insulation savings. We would rather give you a defensible number with the assumptions written underneath it.

How the site pays for itself

The calculators are free and always will be, and none of them require an email address. When you are ready for real prices you can ask us to match you with local builders. That enquiry service is what funds the site. You are never obliged to use it, and the estimate you get is identical either way.

We think that is the honest version of this business model: the tool does not get worse if you decline the upsell.

Where we are

We are based at 2 Killane Road, Ahoghill BT42 1RA. The calculators cover every BT postcode; see areas we cover for the districts, or get in touch.