There is no single price for a house extension in Northern Ireland. Two projects of the same floor area can cost very different amounts, depending on the site, the structure, the finish and what is included in the quote. The figures below are published guide ranges from named sources, not a quote for your house. Use them to sense-check early budgets, then ask local extension contractors and builders for written prices against a clear specification.
Costs vary, and they change as labour and materials move.
Why published ranges differ
Square-metre rates are only a starting point. They are often calculated on different bases: some describe a finished room, some a builder's shell, and some exclude the kitchen, bathroom, VAT or professional fees. Northern Ireland labour rates also tend to sit at the lower end of the UK range, which is one reason a UK-wide average can be a poor fit on its own.
Homebuilding's extension cost calculator explains that labour rates vary significantly across the UK, with Northern Ireland among the lowest, and that its finished-extension rates exclude kitchens, bathrooms, utility rooms, landscaping and VAT. Always ask what a quoted rate includes before you compare it with another.
Guide ranges from named sources
The following published ranges were current when this article was prepared in November 2026. They are not a forecast, and they are not a substitute for local tenders.
RHD Architects (Northern Ireland, 2025 guide figures). In The True Cost of Building a House or Extension, RHD Architects gives rough guide figures for Northern Ireland in 2025 of £1,400 to £2,200 per square metre for extensions. The practice notes that smaller extensions tend to cost more per square metre because fixed items such as foundations and service connections are spread over a smaller area. It also states that these figures usually exclude professional fees (for example an architect, engineer or quantity surveyor), statutory fees (planning and building control) and VAT, where VAT applies.
Checkatrade (UK-wide, not Northern Ireland-specific). Checkatrade's house extension cost guide, accessed in November 2026, quotes a typical finished-extension range of £1,800 to £3,000 per square metre, and a shell-only range of £1,200 to £1,700 per square metre, excluding VAT. It also gives worked examples, including a standard 30 square metre extension at £60,000 to £84,000 excluding VAT. Treat these as UK ballpark figures, not a Northern Ireland price list. Some tables on that page carry different last-updated dates.
McCann Moore (Northern Ireland practice figures, 2026). In What's the 2026 house extension cost in Northern Ireland?, this practice describes typical industry figures it was seeing of roughly £2,000 to £2,800 per square metre for construction costs, excluding kitchen, flooring, landscaping, finishes, VAT, professional fees and contingency. That higher band appears to reflect the architect-led work it typically designs, rather than every straightforward rear extension.
If a source does not give a Northern Ireland figure for the type of extension you want, it is safer to say the cost varies and to obtain local quotes than to invent a number.
What usually sits outside a square-metre rate
A competitive-looking rate can still leave a substantial sum unaccounted for. When you compare quotes, check whether each one includes:
- professional fees for an architect or architectural technician, structural engineer and, on larger jobs, a surveyor
- planning and building control fees, and any specialist reports
- kitchen or bathroom fittings, tiling, flooring and decoration
- new heating, electrics, drainage and utility upgrades
- scaffolding, temporary works and making good to the existing house
- external works, such as paving, steps and garden reinstatement
- VAT, if the contractor is VAT-registered (the VAT treatment of building work depends on the nature of the project; HMRC VAT Notice 708 is the official reference, and you should take advice if you are unsure)
- a contingency for hidden work, often discussed in the region of 10 to 15 per cent for extensions and renovations, though the right allowance depends on the condition of the house
RHD Architects also flags site conditions, design complexity, the level of finish, energy-efficiency measures and market conditions as major cost drivers in Northern Ireland.
What changes the price on a Northern Ireland site
A simple single-storey rear extension on a level, accessible plot in a town such as Belfast or Lisburn and Castlereagh will usually be more straightforward to price than a two-storey addition, a wrap-around, or a build on a tight or sloping site. In more rural council areas, including Mid Ulster and Newry, Mourne and Down, access, drainage and longer service runs can add cost.
Other common variables include:
- opening up a load-bearing wall, which may need a steel beam and an engineer's design
- large glazed openings, bifold or sliding doors, and lantern roofs
- ground conditions, including poor bearing, made ground or the need to divert drainage
- building over or near a public sewer, which can require an agreement with NI Water
- matching older materials, or working on a listed building or in a conservation area
- whether the new space includes a kitchen, bathroom or utility, rather than a simple living room
A garage conversion or a modest side-return is not priced on the same basis as a full new-build extension. If that is the route you are considering, compare like with like and look at loft and garage conversion specialists as well as general builders.
Planning, building control and the order of work
Cost is not only a construction question. Many extensions can be built under permitted development rights if they stay within the legal limits, but others need planning permission. Building control approval is separate and often applies even where planning permission is not needed. See Do I need planning permission for an extension in Northern Ireland? and Do I need planning permission or building control in Northern Ireland?. For formal planning advice, speak to your council or NI Planning Permission.
Getting the drawings, structure and permissions in place before you go out to tender usually produces more reliable prices.
How to use these ranges when you get quotes
- Decide what “finished” means to you: empty plastered shell, decorated room, or ready-to-use kitchen.
- Ask at least two or three extension contractors or builders for written quotations against the same drawings and specification.
- Check insurance, references and a comparable recent job, using the same approach as in How to choose a builder in Northern Ireland.
- Compare exclusions, payment stages and programme, not just the bottom line.
- Allow time and budget for design, permissions and a contingency, then review the figures before you commit.
If you already know the type of work you need, post a job or browse trades in your area, including Belfast and Derry City and Strabane. The only figure that matters is a written quotation from a contractor who has seen the site and the drawings.
Sources
- RHD Architects, The True Cost of Building a House or Extension (2025 Northern Ireland guide figures)
- Checkatrade, How much does a house extension cost? (UK-wide guide, accessed November 2026)
- McCann Moore, What's the 2026 house extension cost in Northern Ireland?
- Homebuilding, Extension cost calculator
- HMRC, VAT Notice 708: buildings and construction