House Extension Cost Per m²: Where the Money Actually Goes (2026)

In Cambridge and the South East, a house extension costs between £2,000 and £3,500 per square metre. That's the short answer. But the long answer — where that money actually goes, what makes one extension cost twice as much as another, and what you're really paying for — is what this guide covers.

House Extension Cost Per m²: The Real Numbers

The most common question we get: "How much per square metre?" Here's what Cambridge homeowners are actually paying in 2026:

Extension Type	Cost Per m²	Typical Total
Single-storey, standard spec	£2,000 – £2,500	£30,000 – £50,000
Single-storey, premium spec	£2,500 – £3,500	£40,000 – £70,000
Double-storey, standard spec	£2,500 – £3,000	£50,000 – £90,000
Double-storey, premium spec	£3,000 – £4,000+	£70,000 – £120,000+
Wrap-around extension	£2,500 – £3,500	£60,000 – £100,000

Figures based on DNDH projects in Cambridge, 2024–2026. Prices include VAT at 20%.

Where Does the Money Actually Go?

This is where most cost guides stop. They give you a per-metre figure and send you on your way. But understanding where that money goes is what helps you make decisions — and control costs.

1. Structure (35–45% of total cost)

This is the most expensive part of any extension. The structure includes foundations, walls, floors, and roof. It's also where costs can spiral if you're not careful.

• Foundations: £80–£150 per m² of floor area. Deeper foundations (for poor soil or near trees) can double this.
• Walls: £100–£180 per m² of wall area. Brick and block cavity walls cost more than timber frame, but last longer.
• Floor slab: £60–£100 per m². Insulated concrete slab is standard; beam and block costs more but allows for suspended floors.
• Roof: £80–£150 per m² of roof area. Flat roofs are cheaper; pitched roofs with tiles cost more but add value.

2. Fit-Out (25–35% of total cost)

This is where specification makes the biggest difference. The same 20m² extension can have £15,000 or £40,000 worth of fit-out depending on your choices.

• Windows and doors: £500–£2,000 per opening. uPVC is cheapest; aluminium and timber cost more but look better.
• Flooring: £30–£150 per m². Carpet to engineered oak — huge range.
• Electrics: £50–£80 per m². More sockets, smart home features, and rewiring add up.
• Heating: £80–£150 per m². Extending your existing system is cheaper than underfloor heating.
• Plastering and finishing: £40–£60 per m² of wall area.

3. Professional Fees (10–15% of total cost)

You can't avoid these, but you can control them. Fees cover:

• Architect: 3–7% of build cost for full service, or £1,000–£3,000 for basic drawings.
• Structural engineer: £500–£2,000 depending on complexity.
• Planning application: £206 householder fee (Cambridgeshire councils).
• Building regulations: £500–£1,500 depending on project size.

4. Groundworks and Services (10–20% of total cost)

Often underestimated. Groundworks include demolition, excavation, drainage connections, and site preparation. Services include new electrical connections, water, gas, and drainage.

• Demolition: £500–£2,000 for outbuildings, conservatories, or internal walls.
• Excavation: £50–£100 per m³. Access issues can double this.
• Drainage: £1,000–£3,000 to connect to mains. More if you need a new soakaway.

5. Contingency (5–10% of total cost)

Always budget for this. Unexpected issues — old foundations, damp, planning conditions — add 5–10% to most projects.

What Affects Your Final Price?

Access

Poor access — no side passage, narrow streets, shared driveways — increases labour time and material delivery costs. A 10m² extension with poor access can cost 20–30% more than the same extension with good access.

Specification Level

The difference between "standard" and "premium" can be £1,000 per m². Premium means: timber or aluminium windows, natural stone or engineered flooring, underfloor heating, high-end kitchen and bathroom fittings, architectural features like vaulted ceilings or rooflights.

Planning

If your extension falls within permitted development, you save time and planning fees. If you need full planning, add 8–12 weeks and £2,000–£5,000 in professional fees. Conservation areas and listed buildings require more detailed applications and can double design costs.

Site Conditions

Clay soil, high water tables, nearby trees, and contaminated land all affect foundation depth and cost. We've seen foundation costs double on sites with poor ground conditions.

What Is the Most Expensive Part of an Extension?

The structure — foundations, walls, floor, and roof — is the most expensive part. It accounts for 35–45% of your total cost. This is why reducing the size of your extension has such a big impact on price: you're not just shrinking the floor area, you're reducing the structure.

After the structure, the next biggest costs are:

1. Fit-out (25–35%) — where your specification choices matter most
2. Groundworks (10–20%) — varies wildly depending on site conditions
3. Professional fees (10–15%) — fixed for most projects

Single-Storey vs Double-Storey: Which Costs More?

Counterintuitively, double-storey extensions cost less per m² than single-storey extensions. Here's why:

• Foundations are already in place for the ground floor
• Roof cost is the same — it just sits higher
• Groundworks and drainage don't increase
• M&E (mechanical and electrical) costs are similar

A 40m² single-storey extension at £2,200/m² = £88,000.
A 40m² double-storey extension (80m² total) at £2,700/m² = £108,000.

You're getting twice the floor area for only 23% more cost.

Extension Costs in Cambridge

Cambridge prices run 10–20% above the national average for several reasons:

• Higher labour costs: Cambridge has a shortage of skilled trades. Good builders are in demand.
• Access issues: Many Cambridge homes have limited side access — terraced houses, narrow plots, shared driveways.
• Conservation areas: Large parts of Cambridge are conservation areas, requiring more detailed planning applications.
• Ground conditions: The clay belt around Cambridge often requires deeper foundations.

Get an Accurate Quote for Your Extension

The figures in this guide are based on real projects we've completed in Cambridge and Cambridgeshire. But every extension is different. The only way to know what your extension will cost is to get a detailed quote.

DNDH Construction provides free, no-obligation estimates within 24 hours. We cover Cambridge, surrounding villages, and the wider Cambridgeshire area.

Frequently Asked Questions