Single-storey extension costs in NI: 2026 price benchmarks

Headline cost ranges for 2026

Three brackets cover almost every single-storey rear extension built in Northern Ireland. These are turnkey figures: design and build through to a clean, decorated, ready-to-use space with a fitted kitchen. They exclude VAT, professional fees and the contingency.

The ranges above reflect quotes from FMB-member builders working across Belfast, Lisburn, Newtownards and the Greater Belfast commuter belt in the first half of 2026, cross-checked against published NI build-cost figures from RHD Architects (£1,400 to £2,200 per square metre for an extension) and Houzz UK NI homeowner surveys. Rural sites, listed properties and off-grid heating set-ups can push individual jobs above these bands; brand-name kitchens (Neff, Miele, Wren higher tiers) will too.

What drives NI cost variance

Two extensions of identical footprint can differ in price by forty per cent or more. The drivers, in roughly the order they bite, are these.

• Ground conditions and foundations. Clay, made-ground, high water table or a sloped site can double groundworks cost overnight. NI has a lot of made-ground along the Lagan and Foyle corridors and clay across the Lough Neagh basin.
• Planning versus permitted development. Permitted development saves the planning fee (around £237 for a full application in NI) and four to eight weeks of wait. Designs that need full planning - conservation areas, AONBs, taller projections - add real cost in redesign rounds and supporting documents.
• Glazing specification. A 4 metre uPVC slider sits around £3,500. The same opening in aluminium bifolds (Origin, Schuco) lands £8,000 to £12,000. The frame is the same shape; the cost is the spec.
• Kitchen and bathroom integration. A knock-through that involves moving a kitchen, adding a utility or fitting a downstairs WC adds plumbing routes, ventilation runs and finishings that an empty extension shell would not.
• Heating compatibility. A rural NI house on oil that needs the tank relocated, or a town house that needs the boiler upgraded to handle the extra demand, can quietly add £2,000 to £5,000 before the extension even has a roof on.

The trade-by-trade cost breakdown

The figures below are NI 2026 ranges for a standard 25 sqm rear extension. They are useful for two things: sanity-checking a builder’s itemised quote, and identifying where your own job sits relative to the band.

Add these together and you land between £39,000 and £97,000 before VAT, professional fees and the contingency - which covers the spread in the turnkey table above. The biggest single line item is almost always the kitchen, followed by glazing. Both are spec-driven, not size-driven, so they are the easiest places to bring a budget in.

NI-specific factors UK guides miss

Most extension cost guides online are written for English homeowners and assume LABC Building Control, mains gas everywhere, and uniform planning rules. None of those hold in NI. The points below are where NI extension projects quietly absorb thousands of pounds that UK-focused guides will not warn you about.

• Building Control sits with your council, not LABC. NI Building Control is administered by each of the 11 local councils, each charging its own fees and operating its own inspection schedule. Belfast City and Lisburn & Castlereagh tend to be the most structured; rural councils sometimes have longer waits for inspections. Budget around £400 to £900 in Building Control fees for a single-storey extension depending on size and council.
• Oil-heating boiler relocation. Roughly two-thirds of NI homes use oil. If the extension footprint overlaps the existing oil tank or boiler, expect £600 to £2,500 to relocate the tank (must be 1.8m clear of openings) and have an OFTEC-registered engineer reconnect the boiler. The cost of disposing of an old tank is extra.
• Septic tank disturbance on rural sites. Homes off the NI Water mains sewer system run on septic tanks or package treatment plants. Foundations close to an existing tank, or new drainage that has to tie into one, frequently trigger a NIEA consent to discharge application or a replacement plant install. The latter runs £4,000 to £8,000 fully fitted.
• Phoenix and Firmus gas connection extension. Greater Belfast is on Phoenix; much of the rest of NI is on Firmus. Both run gas-extension connections rather than the GB national grid. If your extension moves the gas meter or extends an internal run, the relevant network has to do the work, and lead times can run six to twelve weeks. Connection costs typically fall £400 to £1,500 depending on the run.
• NIE Networks supply upgrade. Older NI homes are often on a 60 or 80 amp supply. Adding an electric oven, induction hob, an EV charger and the extension lighting circuit can tip the load over. The upgrade to 100 amps from NIE Networks runs around £800 to £2,500 with several weeks of lead time, and the work must be scheduled with the builder so first-fix electrical does not stall.

Common hidden costs

The cost overrun on an NI extension is rarely the build line items themselves. It is the line items homeowners did not know existed.

• Building Control fees and inspections. Around £400 to £900 to your council, plus the cost of re-attendance if work has to be reopened for a missed inspection.
• Planning application fee (if required). £237 for a householder application in NI as of 2026, plus any architect or planning consultant fees if the design needs revising in response to officer comments.
• Party wall and neighbour agreements. There is no Party Wall Act in Northern Ireland, unlike England and Wales. But where the extension is built up to or against a shared boundary, you still need written agreement from the neighbour and ideally a pre-build condition survey to avoid disputes later. A surveyor report costs £350 to £700.
• Professional fees. Architect or architectural technician at 6 to 10 per cent of build cost (£3,500 to £8,000 typical), structural engineer £500 to £1,500, planning consultant if needed £700 to £1,500.
• VAT at 20 per cent. Almost all extension work is standard-rated. The zero-rate that applies to new-build dwellings does not apply to extensions. Quotes are often given exclusive of VAT; always ask whether the figure includes it.
• Contingency. Ten per cent for a modern, well-built home; fifteen per cent for anything pre-1960 or on a difficult site. This is not optional if you want to finish the project without borrowing more.

Planning versus permitted development in NI

Many single-storey rear extensions in NI fall within permitted development rights, but the thresholds differ from the rest of the UK, and they tighten in conservation areas, AONBs and on listed properties. Always check before you design. The official guidance lives on the nidirect planning portal, and the planning office at your local council will give a free informal view by phone or email before you commit to a full application.

How to get reliable quotes

Treat the figures in this guide as a sanity check, not a quote. Real quotes come from real builders walking the site. A few rules that make those quotes useful.

• Get three written quotes minimum. One quote is a price; three are a market. Two builders within ten per cent of each other and a third twenty per cent below the pair is almost always a sign the cheap quote has missed something. Ask what.
• Insist on like-for-like specifications. Send each builder the same written brief: size, glazing spec, kitchen budget, floor finish, heating set-up. Otherwise you are comparing apples with oranges.
• Tie payments to stages, not dates. Deposit (5-10 per cent), foundations complete (15-20 per cent), walls and roof up (25-30 per cent), first fix (15-20 per cent), second fix and snagging (10-15 per cent), final on sign-off (5-10 per cent). Do not pay for work that is not done.
• Verify insurance and credentials. Public liability, employer’s liability, and any statutory registrations (Gas Safe, OFTEC, NICEIC, FENSA) on the day, on the relevant public register.

We cover the full quote-comparison and contracts process in depth in our guide to hiring a builder safely in NI - it is the natural companion to this cost benchmark and covers stage payments, written contracts and the warning signs that turn an extension into a dispute.

Frequently asked questions