How Much Does A Single Storey Extension Cost?

The National Average Cost (And Why It’s Just A Starting Point)

As of mid-2025, the average cost of a single storey extension in the UK ranges between £1,800 and £2,800 per square metre, depending on the spec. That includes labour, materials, basic fittings, and project management - but not VAT or planning fees.

So a modest 4m x 5m extension? You’re likely looking at somewhere between £36,000 and £56,000 before add-ons.

But averages are just that - averages. Go high-spec, build in London, or deal with tricky access, and your quote could easily push beyond £3,500/m². On the other hand, a simple rear kitchen extension in the Midlands with no structural surprises might sit well below £2,000/m².

What Drives The Cost Up (Or Down)?

It’s not just the size that matters. Cost is driven by complexity. And that includes…

1. Foundations And Ground Conditions

Building on a flat, clear site with good drainage is one thing. But if your soil needs reinforcement, or if there’s any kind of tree root management or excavation involved, costs rise sharply. Think mini-piling, reinforced slabs, or underpinning - all of which add thousands.

2. Structural Work And Knock-Throughs

Adding new walls is relatively straightforward. But if your extension design includes knocking through to create an open-plan space, you’ll need RSJs (steel supports), calculations from a structural engineer, and potentially even temporary supports. Not only does this push the labour and material costs higher, but it lengthens the build time.

3. Glazing Choices And Openings

Natural light makes a huge impact - but big bi-folds and roof lanterns aren’t cheap. You can absolutely get quality without overspending, but premium aluminium frames, triple glazing, and architectural rooflights can push glazing costs into five figures.

Want something that looks great without the steep markup? Many homeowners choose to shop high-quality roof windows that balance light, insulation, and aesthetics - without wrecking the budget.

4. Finishes, Fixtures, And Fit-Out

You could leave the interior as a blank shell. Or you could finish it with underfloor heating, bespoke joinery, Italian tiles, and designer lighting. The gulf between basic and premium fit-outs is enormous - sometimes bigger than the build cost itself.

As a rough guide:

• Basic finish: £5,000–£8,000
• Mid-range: £10,000–£20,000
• High-end: £25,000+

And that’s not including the cost of kitchens, bathrooms, or furniture.

What’s Not Included In Most Quotes?

This is where things get sticky. Many quotes look lower than expected - but only because key items are left out. Always ask what’s excluded.

Common extras include:

• Planning permission (if required)  -  ~£250
• Building regulations approval
• Party wall agreements
• Architect fees (often 7–15% of the project cost)
• Structural engineer reports
• Waste removal and site clearance
• Contingency for unexpected problems (always budget 10–15%)

So even if your base quote is £45,000, don’t be surprised if the full cost climbs towards £60,000 once everything is accounted for.

Rear Vs Side Vs Wraparound Extensions

The layout of your property - and the type of extension you choose - can make a massive difference to cost.

• Rear extensions are usually the cheapest: straight-forward builds, minimal disruption.
• Side extensions can be trickier depending on boundaries and access.
• Wraparounds (rear + side) combine the complexity of both and usually cost 40–60% more than a basic rear-only design of the same depth.

Can You Extend Under Permitted Development?

Yes - sometimes. If your proposed extension meets certain criteria (usually size, height, and location), you may not need full planning permission. This saves time and application costs.

But even under permitted development, you’ll still need:

• Building regs approval
• Compliance with party wall legislation
• Professional drawings and calculations

In short, “PD” doesn’t mean “easy” - but it can simplify the process.

Ways To Keep Costs Under Control

It’s not about cutting corners. It’s about cutting waste - and knowing what adds value.

• Choose standard dimensions. Custom sizing adds labour and material waste.
• Limit structural changes. Fewer knock-throughs mean fewer steel supports.
• Use off-the-shelf materials. Bespoke always costs more, often without adding resale value.
• Phase your project. Can you build the shell now and fit out later?
• Get fixed-price quotes. Avoid day rates unless you trust the builder implicitly.

Don’t Forget The Impact On Property Value

It’s easy to get tunnel vision around budgets and deadlines - but don’t lose sight of the long game. A well-designed, sympathetically built single storey extension can add up to 20% to your home’s value, particularly if it enhances key living areas like kitchens or open-plan lounges. That said, the return on investment varies wildly depending on location, build quality, and market conditions. Extensions that feel “bolted on” or clash with the existing architecture may add little - or even reduce resale appeal. So it’s worth thinking not just about what works now, but what will still make sense if you sell in five years.

How Long Does A Single Storey Extension Take?

Timelines vary, but here’s a typical breakdown:

• Design & Planning: 4–8 weeks
• Planning Permission (if required): 8–12 weeks
• Pre-construction (drawings, engineer reports, party wall notices): 2–4 weeks
• Build Time: 10–16 weeks
• Fit-out & Snagging: 2–4 weeks

So from idea to completion? Allow 4–6 months minimum. Double that if your local authority moves slowly or your design is complex.

Final Thoughts

So, how much is a single storey extension?

It depends. But more than that, it’s not just about price - it’s about planning. A well-managed £60k build that runs on time, delivers the space you need, and adds long-term value is always better than a £40k one that cuts corners and causes problems later.

And while average figures are a good starting point, your final cost will reflect the decisions you make at every stage - materials, layout, finishes, and project management.

Budget smart, ask the right questions early, and be honest about where you can compromise. That’s how you keep control - not just of cost, but of the entire process.