How Much Does a House Extension Cost? A Realistic Guide
If you’ve started researching house extension costs online, you’ve probably already encountered a frustrating range of figures. One site tells you £1,500 per square metre. Another says £3,000. An Instagram post from someone who “just finished their extension” quotes something else entirely.
The truth is that all of those figures can be correct - and none of them is particularly useful on its own. The cost of a house extension depends on so many variables that headline numbers without context are almost meaningless. This guide is an attempt to give you something more honest: a realistic picture of what drives costs, what you should actually budget for, and how to think about your project before you’ve spoken to anyone.
What actually drives the cost of a house extension?
Before we get to numbers, it’s worth understanding the factors that move the dial most significantly.
Size and footprint
This is the obvious one. More floor area means more materials, more labour, more time. But size alone doesn’t determine cost — a small extension with a complex brief can easily cost more than a larger, simpler one.
Structural complexity
A single-storey rear extension on flat ground is a very different proposition to a double-storey side extension that requires underpinning, a new staircase, and significant roof work. The more structurally complex the project, the higher the cost per square metre tends to be.
Specification and finish
This is the factor that most online cost guides underplay, and it’s arguably the most important. The difference between a standard specification and a high-quality one - in terms of materials, windows, insulation, joinery, and fittings - can easily double the cost of a project. A basic rear extension and a beautifully considered one might have identical footprints but wildly different price tags.
Location
Construction costs vary significantly across the country. In the South West, costs are generally closer to the national average than London and the South East - but in desirable areas like Bath and Bournemouth, demand for experienced contractors is high, and that affects pricing. In our experience working across the region, budget assumptions based on national averages often need adjusting upward for well-specified projects in these areas.
Who you hire and how you manage the project
An architect-led project with a specialist contractor will cost more than a design-and-build package - but it will also typically deliver a better outcome, fewer surprises, and a more considered result. We’ll come back to this.
Rough costs by extension type
These figures represent realistic current costs for well-specified projects. They include construction costs but not professional fees, planning, or fit-out - more on that shortly.
Single-storey rear extension
The most common type of extension. A straightforward single-storey addition might start from around £2,500 per square metre for a good quality finish. For a 30m² extension, that’s £75,000 before planning, building regs, or consultant fees. For a higher specification - better materials, larger glazing, more complex detailing - costs of £3,000 - £4,000 per square metre are not unusual. A 30m² extension at this level would be £90,000 - £120,000. Typically, we are seeing modest rear extensions around the £100,000 mark.
Double-storey extension
Adding two storeys is not twice the cost of one - the foundation and roof work is shared - but it is significantly more complex. Expect to add 50–70% to a comparable single-storey budget.
Side return extension
Common in Victorian terraced houses, side return extensions are often relatively modest in size but can make a huge difference to your house. Costs are similar per square metre to a rear extension, though the narrower working space down the side can add complexity.
Larger and more ambitious schemes
Wraparound extensions, basement conversions, or projects that combine an extension with a significant internal reconfiguration are a different category entirely. These projects - particularly when architect-led and highly specified - typically start from £300,000 and can go considerably higher depending on ambition and complexity.
What most quotes don’t include
This is where many homeowners get caught out. Construction costs are only part of the picture. A realistic budget for an extension project also needs to account for:
Professional fees
Architect fees for a well-managed project typically run at 10–15% of construction cost. You’ll also need a structural engineer, and depending on the project, potentially a planning consultant, party wall surveyor, and others. On a £200,000 construction project, professional fees alone might be £25,000–£35,000.
Planning and building regulations
Planning application fees are relatively modest, but the time and cost involved in preparing drawings and supporting documents is not. Building regulations approval is a separate process and cost.
VAT
Construction work on existing homes is subject to 20% VAT in most cases. On a significant project, this is a substantial sum that is easy to overlook when comparing quotes.
Landscaping and external works
Extensions almost always affect gardens, drainage, and external areas. If you want to enjoy your completed project as soon as possible, make sure you budget for landscaping from the get-go. Alternatively, make a plan to take on the landscaping as a DIY project once the build is finished.
Interior fit-out
Kitchens, bathrooms, flooring, joinery, decoration - these costs can be significant and are typically excluded from construction quotes entirely.
When you add all of this together, the real cost of an extension project is often 30–40% higher than the headline construction figure. A project quoted at £150,000 to build might realistically cost £200,000 or more to complete to a finished standard.
Why the cheapest quote is rarely the best value
It’s natural to want to keep costs down as much as possible. But in construction, the relationship between price and quality is closer than in almost any other sector - and the consequences of getting it wrong are significant.
A contractor who prices low to win work may cut corners on materials, use less experienced labour, or find ways to recover margin through changes and extras once the project is underway. A project that starts at a promising price can quickly become more expensive, more stressful, and slower than one that was properly costed from the start.
More importantly, a well-designed and well-built extension is a long-term investment. The difference in cost between a good project and an exceptional one is often surprisingly small as a proportion of the overall spend — but the difference in the result, and in how much you enjoy living in it, can be enormous.
At ReFrame Studio, we start every project by taking the time to understand what matters most to you — how you live, what you need from your home, and where the budget will have the greatest impact. That means money gets spent in the areas that will genuinely serve you, rather than on whatever happens to be all over Pinterest this season.
How to set a realistic budget
A few principles that apply to almost any extension project:
• Build in contingency from the start. Even well-managed projects encounter the unexpected — particularly in older properties where you don’t fully know what you’re working with until you start. A contingency of 15–20% of construction cost is sensible; treat it as part of the budget, not a buffer you hope not to use.
• Get proper cost advice early. Early cost plans can help give you a realistic picture as you go, so you can avoid disappointment when the quotes come in. Your architect will be able to arrange a cost plan for you and advise on the best time to invest in one for your specific project.
• Think about total cost, not just build cost. As outlined above, construction is only one element. Understanding the full picture early prevents unpleasant surprises later.
• Consider the value of experience. For a straightforward extension, the market is competitive and there are many capable contractors. For a more complex, ambitious, or high-value project, the experience of your team — architect, contractor, project manager — becomes a much more significant factor in whether the project is a success.
Thinking about your next step?
If you’re in the early stages of planning an extension in Bath, Bournemouth, or the surrounding area, the most valuable thing you can do before approaching contractors or architects is to get clear on what you’re trying to achieve - and what a realistic budget looks like for that ambition.
At ReFrame Studio we offer a free initial consultation for serious projects. It’s a no-obligation conversation about your plans, your priorities, and whether we’re the right fit for each other.
Book a free consultation or get in touch by email - we’d love to hear about your project.
About ReFrame Studio
ReFrame Studio works on substantial residential projects across the South West — extensions, renovations, and new builds in Bath, Bournemouth, and the surrounding area — typically from £250,000 upwards.