When it comes to building a new home or extending an existing one in Northern Ireland, one of the first questions clients ask is: “How much will it cost?” It’s a fair question - after all, building or extending a home is likely the largest investment most people will ever make. But while it’s tempting to look for a quick price-per-square-foot answer, the reality is far more complex.

The true cost of building a house or extension is shaped by a wide range of factors - from site conditions and planning permission requirements to design choices, sustainability measures, and even the current state of the construction market.

In this post, I’ll break down the key elements that determine cost, highlight common misconceptions, and share practical advice to help you budget realistically for your project in Northern Ireland.

1. The Myth of a Fixed Price per Square Foot

A quick internet search will throw up average build costs ranging from £1,500 to £2,500 per square metre for new homes in Northern Ireland, and around £1,200-£2,000 per square metre for extensions. While these figures can provide a starting point, they are often misleading.

Why? Because no two sites, homes, or clients are the same. A flat, accessible plot with straightforward ground conditions will cost less to build on than a steep, rural site requiring retaining walls, specialist drainage, or long service runs. Similarly, a simple single-storey rear extension will cost less than a two-storey addition with complex steelwork and high-spec finishes.

The takeaway: use square-metre rates cautiously, and be prepared for your actual costs to vary.

2. Factors That Influence Build Costs in Northern Ireland

Let’s look at some of the biggest cost drivers you’ll need to consider:

Site Conditions – Ground quality, access for machinery, existing services, and whether the site is in a rural or urban location.
Planning Permission & Regulations – Projects in conservation areas, near listed buildings, or in sensitive rural locations may require additional reports, specialist materials, or design adaptations.
Design Complexity – Bespoke details, large areas of glazing, vaulted ceilings, or unusual shapes add cost compared to simpler layouts.
Level of Finish – Kitchens, bathrooms, flooring, and bespoke joinery can account for a significant portion of the budget. A “builders’ finish” is very different to a high-end interior.
Sustainability & Energy Efficiency – Features such as triple glazing, airtight construction, MVHR systems, or Passivhaus-standard insulation add upfront cost, but they pay back in long-term savings and comfort.
Market Conditions – Material prices and labour availability fluctuate. Post-pandemic, Northern Ireland has seen spikes in timber, steel, and insulation costs.

Some of the associated costs in building a house

3. Typical Cost Ranges

While every project is unique, here are some rough guide figures for Northern Ireland in 2025:

New build homes: £1,700 – £2,400 per m² (depending on complexity and finish).
Extensions: £1,400 – £2,200 per m² (smaller extensions tend to be more expensive per square metre because of fixed costs like foundations and connections).
Renovations & Conversions: Highly variable — sometimes more expensive than new build due to hidden issues in existing structures.

Remember: these figures usually exclude professional fees (architect, engineer, QS), statutory fees (planning, building control), and VAT (if applicable).

4. Budgeting Beyond the Build

When planning your budget, don’t forget the “invisible” costs that can easily be overlooked:

Professional Fees – Architect, structural engineer, services engineer, and possibly a quantity surveyor. An architect’s involvement often saves money in the long run through better design, reduced errors, and smoother approvals.
Statutory Fees – Planning applications, building control submissions, utility connections, and land registry fees.
Contingency – A healthy contingency (typically 10–15%) is essential to cover unforeseen issues. Few projects ever run without surprises, particularly extensions and renovations.
Furnishings & Landscaping – Often left out of initial budgets but essential to creating a finished home.

5. Value vs. Cost

One of the most important things to remember is that cheapest is not always best. Cutting corners on design, insulation, or quality of materials may reduce upfront spend, but it often results in higher running costs, maintenance headaches, and a home that doesn’t serve you well long-term.

An investment in thoughtful design and energy efficiency creates a home that is comfortable, adaptable, and cheaper to run for decades. In Northern Ireland, where energy prices remain high, this has never been more important.

6. How an Architect Helps Manage Costs

A good architect doesn’t just draw plans - they help clients navigate the financial realities of building. At RHD Architects, we:

Develop designs that balance aspiration with budget.
Advise on where to invest for long-term value and where savings can be made.
Coordinate early cost advice from builders or quantity surveyors.
Manage planning and building control applications to avoid costly delays.
Help you phase work if required, spreading costs over time.

In other words, we help you build smarter, not just bigger.

Discussing your scheme with an architect at an early stage

7. Final Thoughts

Building or extending a home in Northern Ireland is a significant undertaking, both financially and emotionally. By understanding the true costs involved - not just construction, but also professional fees, regulations, and long-term value - you can make better-informed decisions.

Every client’s journey is different, but the best outcomes come when budgets are realistic, expectations are clear, and design is seen as an investment, not an expense.

If you’re considering a new build or extension in Northern Ireland, the best first step is a conversation. By talking through your aspirations, site, and budget, we can help you plan a project that delivers a home you’ll love - and one that works financially. If you’re considering a project and would like to discuss your ideas, feel free to get in touch.

The True Cost of Building a House or Extension