How Much Does a House Extension Cost in Belfast in 2026?

Almost every extension cost article you will find online is written for the UK as a whole, and most of them are written for the south east of England. Northern Ireland is one of the cheapest places in the UK to build. If you budget from a national average you will either scare yourself out of a project you can afford, or you will assume every local quote is a rip-off because it does not match a number that was never meant for Belfast.

This guide gives the figures we actually use, how they are built up, and what pushes a job to the top of the range.

The short answer: £1,400 to £2,200 per square metre

For a single-storey extension in Belfast in 2026, budget £1,400 to £2,200 per square metre of finished floor area for the construction itself. Mid-specification work — the finish most people end up with — sits at £1,700 to £2,200 per m².

Those are VAT-inclusive rates. Building work to an existing home is standard-rated at 20 per cent, and builders here quote domestic customers inclusive, so the number a builder writes down is the number you pay.

The rate is not the whole budget. On top of the construction cost you need professional fees and a contingency, which is roughly another 20 per cent between them. So a 25m² extension at £2,000/m² is £50,000 of building work and somewhere around £60,000 all-in.

What the rate covers

The per-square-metre rate assumes a complete, finished room you could move furniture into:

• Site set-up, excavation, foundations and the ground-floor slab
• Drainage connections and any diversions
• Blockwork, cavity insulation, roof structure and roof covering
• Windows, external doors, fascia, soffit and rainwater goods
• First and second fix electrics and plumbing to a normal standard
• Plaster, screed or floor build-up, internal doors, skirting and decoration

It does not cover a kitchen, a bathroom, bi-fold doors, underfloor heating, or knocking a large structural opening through into the existing house. Those are all real costs and they are all separate. See the extras table further down.

A worked example: a 25m² rear extension in south Belfast

A 5m × 5m rear extension, standard specification, level garden, decent side access.

Strip out the extras and you are at roughly £51,700 to £65,500 for a plain, finished 25m² box. Add a kitchen and bi-fold doors and you are comfortably into the seventies.

For a general planning figure, a 20 to 30m² rear extension in Northern Ireland lands between £34,000 and £66,000 all-in, depending on size, specification and how awkward the site is. That is a wide range, and anyone who gives you a narrower one without seeing the house is guessing.

A range is not indecision. Two identical-looking extensions on the same street can differ by twenty thousand pounds because one has a shared entry, a mature sycamore near the foundations and a wall that turns out to be single-skin.

What moves the number

Size works against small extensions

Foundations, drainage runs, the junction between the new roof and the old one, the scaffold and the site set-up all cost much the same whether the extension is 12m² or 30m². Spread over a small floor area they hurt. Expect a 12m² extension to cost around 15 per cent more per square metre than a 40m² one. Going bigger is rarely as expensive as people fear — the second half of an extension is always cheaper than the first.

Access

If a mini-digger and a dumper can get to the back of the house, the groundworks are quick. If everything has to come through the hall in buckets, or through a shared entry off an alleyway, add roughly 8 per cent. If the only option is a crane over the roof, add 15 per cent and be glad it is possible at all. Terraced streets in south and east Belfast are where this bites hardest.

Ground conditions

Level garden and ordinary subsoil is the baseline. A sloping site needing stepped foundations adds around 6 per cent. Made ground, a high water table or a mature tree within influencing distance can add 10 per cent or more, and occasionally a great deal more if the engineer specifies piles. If you do not know what is under the garden, allow for it rather than hoping.

Specification

Basic means a functional room: uPVC windows, a standard roof covering, painted plasterboard. High specification means aluminium glazing, a flat roof with a large structural rooflight, engineered timber or a good-quality tile through the floor, and detailing that takes time. The gap between the two is around £700 per square metre, which on a 25m² extension is £17,500. Specification is the single biggest lever you control.

The extras, priced for Belfast

These are the items people forget when they compare quotes. Belfast rates, VAT included.

If a quote looks suspiciously cheap, this table is usually where the difference is hiding. One builder has priced the kitchen and the other has assumed you are supplying it.

Reinstatement outside is worth pricing separately too, because a build leaves the ground around it churned up and the machinery has to come in over something. If the driveway is going to need relaying afterwards, our comparison of block paving and tarmac in Northern Ireland has 2026 rates per square metre.

Fees, contingency and the statutory costs

Allow 10 per cent for professional fees. That covers drawings, a structural engineer for any beam or unusual foundation, and the planning and building control applications. A small, simple extension might come in under it; anything with a steel and a large opening will not.

Allow 10 per cent contingency. Not because your builder is unreliable, but because nobody knows what is under the ground or inside the wall until it is opened up. If you do not spend it, you have a new kitchen floor.

On top of that there are statutory fees. A householder planning application in Northern Ireland runs to a few hundred pounds, and building control is charged separately by your district council on a scale linked to the value of the work. Both are set by regulation and both change, so ring your council rather than trusting a figure you read anywhere, including here. There is more on how the two systems differ in our guide to building control in Northern Ireland, and on when you need consent at all in planning permission for an extension in NI.

How Belfast compares

Northern Ireland runs roughly 15 to 20 per cent below the UK average for construction, and the gap is almost entirely labour. The same 25m² extension, same specification:

That is why a national calculator is the wrong tool here. Our extension cost calculator applies the multiplier for your postcode, so BT9 gets Belfast prices rather than a blended UK figure.

Cheaper ways to get the same space

Before you commit to an extension, price the alternatives. A loft conversion typically costs less per square metre because the roof, the walls and the foundations already exist. A garage conversion is cheaper again — often under £18,000 for a room you already own the shell of. Neither suits every house, but both are worth ten minutes of your time before you spend sixty thousand pounds.

What a fair quote looks like

A quote you can act on is broken down. It states the specification, names the windows and the roof covering, separates the kitchen from the building work, lists any provisional sums, and sets out a payment schedule tied to stages of the job. A single number on one page tells you nothing and protects nobody.

Get three of them, priced against the same drawings and the same written specification, and ignore the outlier in both directions. Our guide on how to choose a builder in Belfast sets out the twelve questions worth asking before you sign anything.

Rates on this page were last reviewed in August 2026. They are indicative estimates for planning purposes, not a quote.