Planning a house extension is exciting. Working out how much it’s going to cost is where things can quickly get complicated.
This guide covers realistic 2026 cost ranges for every type of residential extension, the key factors that affect what you’ll actually pay, and how to make sure your budget is based on something more reliable than an online calculator.
| Extension Type | Cost Range (£/m²) | Typical Total Cost |
|---|---|---|
| Single-storey rear | £1,800 – £3,200/m² | £25,000 – £80,000+ |
| Double-storey | £1,600 – £2,800/m² | £40,000 – £120,000+ |
| Side return | £1,900 – £3,000/m² | £20,000 – £60,000 |
| Wrap-around | £2,000 – £3,500/m² | £60,000 – £150,000+ |
| Orangery / garden room | £1,500 – £2,500/m² | £15,000 – £60,000 |
| Basement conversion | £2,500 – £4,000+/m² | £40,000 – £150,000+ |
and VAT. Your actual cost will depend on location, specification, ground conditions, and contractor. A professional estimate is the only way to get a reliable figure for your specific project.
The most common type of extension in the UK. A single-storey addition to the rear of a property is typically used to create an open-plan kitchen-diner or add a utility room.
Costs increase with higher specification finishes, bi-fold or sliding doors, roof lanterns, underfloor heating, and structural complexity.
Adding two floors increases usable space significantly without proportionally increasing costs – because you’re sharing the foundation, roof, and scaffolding across two levels. This makes a double-storey extension more cost-efficient per square metre than a single-storey build.
Planning permission is almost always required for a double-storey extension, so factor in planning fees and potential delays.
Side return extensions are popular in Victorian and Edwardian terraced properties where a narrow gap alongside the kitchen can be incorporated into the main living space.
Steel work is often required to create an open-plan layout, which adds to cost. Party wall agreements with neighbours may also be needed.
A wrap-around combines a rear and side return extension into one L-shaped structure. It’s one of the most transformative ways to extend a home but also one of the most expensive – costs reflect the increased complexity, multiple structural elements, and larger footprint.
The figures above give a starting point, but your final cost will be shaped by a range of factors. Understanding these helps you budget more accurately and avoid surprises during the build.
Labour costs vary significantly across the UK. London and the South East typically attract a 15–25% premium compared to the national average. The North of England and parts of Wales and Scotland often come in 10–15% below average rates.
A standard extension with off-the-shelf fittings costs considerably less than one with bespoke joinery, premium kitchen appliances, large-format tiles, and architectural glazing. Specification is often where costs spiral beyond the initial budget.
If the ground requires significant excavation, sheet piling, or specialist foundations – perhaps due to tree roots, clay soil, or poor-quality ground – costs can increase substantially. A soil survey before planning can help flag issues early.
Difficult site access adds cost. If scaffolding has to be erected from a neighbouring property, or if materials can only be delivered by hand, your prelim costs will be higher.
Party wall agreements, planning applications and building regulations submissions all carry professional fees and can extend the pre-construction timeline. Build these into your overall project budget, not just the build cost.
Removing load-bearing walls, installing steel beams, and underpinning foundations all add cost. A structural engineer’s fees and the cost of the steelwork itself should be included in any serious estimate.
Many online calculators give you a cost-per-m² figure without being clear about what’s excluded. Before relying on any estimate, make sure the following are accounted for:
| Region | Typical Adjustment vs National Average |
|---|---|
| Inner London | +20% to +30% |
| Outer London / Home Counties | +10% to +20% |
| South East | +5% to +15% |
| South West / East of England | ±5% |
| Midlands | -5% to +5% |
| North of England | -10% to -15% |
| Wales / Scotland | -10% to -20% |
Online cost calculators have their place – they’re a reasonable starting point for ballpark thinking. But they work from averages, they can’t account for your specific ground conditions, access, spec choices or local labour rates, and they don’t produce anything you can put in front of a mortgage lender, planning consultant or building contractor.
For a project you’re actually going to build – or quote for – you need an estimate based on your drawings and your specification.
The most common reason extension projects go over budget isn’t unexpected problems during the build. It’s that the original budget wasn’t based on a proper estimate in the first place.
Get an accurate estimate for your extension project with ProQuant.
With a strong foundation built from hands on site experience in his early career, Oliver Wilcox brings a practical and informed perspective to the construction industry. He went on to earn a BSc (Hons) in Building Studies, further strengthening his technical expertise and understanding of the built environment.
Following this, he spent 10 years working within the estimating sector, developing a deep knowledge of cost planning, measurement and project evaluation across residential developments.
In 2011, he co-founded Proquant Estimating LTD alongside his business partners, with a vision to deliver affordable, accurate, efficient and reliable estimating services.
Since then, the company has grown significantly and is recognised as the leading residential estimating service throughout the UK.
His combined site experience and professional expertise continues to drive Proquant’s commitment to precision, quality and client focused delivery.
