How we work these figures out

An estimate is only worth anything if you can see how it was built. This page sets out the whole method, the numbers behind it, where they came from and when they were last checked.

The formula

Every cost calculator on this site works the same way. There is no black box:

Floor area × base rate per m² × build type × size factor × access factor × ground factor × regional multiplier, plus itemised extras, plus professional fees, plus contingency.

1. The base rate

Base rates are held per calculator, per specification tier, as a low-to-high band in pounds per square metre. They are UK-average figures and they are inclusive of VAT at 20 per cent, because work to an existing home is standard-rated and Northern Ireland builders normally quote domestic customers VAT-inclusive. Quoting you an ex-VAT figure would understate your budget by a fifth.

2. The regional multiplier

This is the part most national calculators get wrong for anyone building here. Labour is the largest variable in a building project and it does not cost the same across the UK. Your postcode resolves to a region, and the region carries a multiplier against the UK average.

3. The size factor

Small jobs cost more per square metre. Foundations, drainage connections, roof junctions and the cost of simply getting a team to site are largely fixed, so they are spread over fewer square metres. A 10m² extension is not half the price of a 20m² one.

4. Access and ground

If a digger cannot reach the back of the house, the groundworks are dug by hand. That is slow, and slow is expensive. Sloping sites need stepped foundations. Made ground, a high water table or mature trees nearby all mean going deeper. These are multipliers, not afterthoughts, because on a real site they are what moves the number.

5. Professional fees and contingency

Ten per cent for professional fees covers drawings, structural calculations, building control and planning submissions. Ten per cent for contingency is the working minimum, and fifteen is wiser on an older property. Contingency is not padding: it is what stops a drainage diversion turning into a stalled job and a dispute.

The regional multipliers in full

1.00 is the UK average. Anything below it is cheaper than the UK average, anything above is dearer.

Region	Multiplier	vs UK average	Notes
Belfast & Greater Belfast	0.84	-16%	Belfast and the commuter belt sit a little above the wider Northern Ireland average, mainly on labour and access.
Northern Ireland	0.82	-18%	Northern Ireland is consistently one of the most affordable parts of the UK to build in — roughly 15–20% below the UK average.
Scotland	0.92	-8%	Broadly below the UK average, though the Highlands and islands carry a delivery premium.
North East England	0.90	-10%	One of the cheaper mainland regions for construction labour.
North West England	0.93	-7%	Below the UK average, with Manchester and Cheshire pulling upwards.
Yorkshire & the Humber	0.93	-7%	Below the UK average; Harrogate and York run higher than the regional norm.
Wales	0.92	-8%	Below average overall, with a premium for rural and mountain access.
East Midlands	0.95	-5%	Slightly below the UK average.
West Midlands	0.97	-3%	Close to the UK average.
South West England	1.03	+3%	Slightly above average, rising sharply in Bath, Bristol and coastal Cornwall.
East of England	1.05	+5%	Above average, strongly influenced by the London commuter belt.
South East England	1.12	+12%	Well above the UK average on both labour and materials delivery.
London	1.32	+32%	The most expensive UK region; prime central boroughs run higher still.
Channel Islands & Isle of Man	1.20	+20%	Shipping and a small contractor pool add a significant premium.

The insulation calculators are different

The insulation tools do not just multiply an area by a rate. They model the building physics: thermal resistances are added in series with the standard surface resistances from BS EN ISO 6946, the existing element starts from the same default U-value a reduced-data SAP assessor would use when they cannot open up a wall, and the result is derated for thermal bridging through joists and studs, because a real installation never achieves the plain layered calculation.

Savings come from a degree-day calculation using a Northern Ireland heating season, your heating fuel and its efficiency, and a utilisation factor of 0.70. That last number matters: households bank roughly seventy per cent of the theoretical saving, because the rest is taken as comfort. People heat rooms they used to leave cold. Ignoring that is the main reason published saving figures do not survive contact with a real energy bill.

One thing we will never do: report an improvement to a single wall or ceiling as though it were a reduction in your whole heating bill. An eighty per cent cut in heat transmission through one element is not an eighty per cent cut in what you spend on oil.

Sources

Rates on this site are built from:

Belfast builders merchant cement and aggregate prices, 2026
Belfast builders merchant panel, post and postmix prices, 2026
Belfast builders merchant paving supply prices, 2026
Belfast roofing contractor quoted rates, 2026
Belfast worktop fabricator supply and fit rates, 2026
BS EN ISO 6946: surface resistances and the layered U-value method
Building Regulations (Northern Ireland), Technical Booklet F1: Conservation of fuel and power in dwellings
Northern Ireland builders merchant brick, sand and cement prices, 2026
Northern Ireland builders' merchant and DIY retail prices for loft quilt, 2026
Northern Ireland fencing contractor rates, 2026
Northern Ireland garage conversion cost guides, 2026
Northern Ireland garden room and garden office supplier pricing, 2026
Northern Ireland kitchen supplier and fitter pricing, 2026
Northern Ireland landscaping and paving contractor rates, 2026
Northern Ireland loft conversion cost guides, 2026
Northern Ireland paving and surfacing contractor rates, 2026
Northern Ireland ready-mix batching plant prices, 2026
Northern Ireland roof replacement price guides, 2026
Northern Ireland single-storey extension cost guides, 2026
Quoted bricklaying rates from Belfast and Greater Belfast, 2026
Quoted installer rates from Belfast and across Northern Ireland, 2026
Quoted rates from Belfast and Greater Belfast contractors, 2026
Quoted rates from Belfast and Greater Belfast installers, 2026
Quoted rates from Belfast and Greater Belfast loft specialists, 2026
Quoted supply-and-fit rates from Northern Ireland insulation installers, 2026
RdSAP default U-values for existing wall constructions
SAP and RdSAP conventions for default U-values, and the degree-day method for heat loss
Standard UK brick dimensions and mortar joint conventions
Standard UK concrete mix designs and yields
Tobermore and locally manufactured block paving supply prices, 2026
UK regional build cost indices, Q3 2026
Building Regulations (Northern Ireland), Technical Booklet F1, for insulation targets.
UK government greenhouse gas conversion factors, for the carbon figures.

What these figures are not

They are not a quotation, and no estimate produced here forms any kind of offer. Two builders can look at the same job and price it thirty per cent apart, and both can be right, because they are pricing different risk, different programme and different standards of finish.

Use the estimate to sanity-check a quote, to set a budget, and to know when a price is worth questioning. Then get three written quotations.

Found something wrong?

If you have quoted or delivered a job in Northern Ireland recently and our figure is off, tell us. Real quotes are how these rates get better. Send us the detail and we will look at it.

All rates on this site were last reviewed on 1 August 2026. Every calculator shows its own review date at the top of the page.