How Much Does It Cost to Build a House Extension in 2026?

£/m² Building Prices  ·  London & UK  ·  Updated April 2026

If you are planning a home extension in London, one of the first questions you will ask is: how much is this going to cost? It is also one of the most difficult questions to answer precisely — because no two extension projects are the same.

At Excela Architecture London, our home extension architects work with clients across London at every stage of the design and planning process. From our experience delivering extension projects across the capital, we know that construction costs are shaped by a wide range of variables — from the size and structural complexity of your extension to the quality of materials and finishes you choose.

This guide gives you an honest, up-to-date picture of house extension costs per square metre in the UK and London in 2026, along with the key factors that will determine your true budget. Whether you are in the early ideas stage or ready to move forward with your planning application, the figures below will help you plan with confidence.

"Building costs in London are among the highest in the world — and understanding what drives those costs is the first step to protecting your budget."

2026 benchmark figures

House Extension Cost per m² in the UK

UK average (entry)

£1,750

per m² — basic specification

UK average (mid)

£2,300

per m² — standard specification

London premium

£2,500

per m² — typical London build

London high-end

£3,500+

per m² — premium specification

Estimated Costs by Extension Type (London, 2026)

The table below gives realistic indicative cost ranges for common home extension types in London. These figures cover construction only and do not include architect fees, structural engineering, planning fees, or VAT.

Extension type	Typical size	Estimated cost range
Single-storey rear extension	15–30 m²	£37,500 – £90,000
Double-storey rear extension	30–50 m²	£80,000 – £175,000
Side return extension	10–20 m²	£30,000 – £70,000
Wraparound extension	25–45 m²	£75,000 – £160,000
Over-structure / roof extension	20–40 m²	£70,000 – £140,000

All figures are indicative benchmarks based on London market conditions in 2026. Your actual cost will depend on the specific design, structural requirements, and specification of your project. The most accurate way to establish your build cost is through detailed drawings and a bill of quantities — something your architect and structural engineer will help produce.

What Additional Costs Should You Budget For?

Your construction cost is just one part of the overall investment. When planning a home extension budget, you should also account for the following:

Budget allowances beyond build cost

Architect & structural engineer fees~15% of build cost

Planning application & statutory consents~3–5% of build cost

Building regulations approval£1,000 – £3,000+

Contingency (unforeseen site conditions)~10% of build cost

VAT (on contractor invoices)20% (standard rate)

The Three Factors That Determine Your Extension Cost

In our experience as home extension architects in London, three factors have the greatest influence on what your project will ultimately cost.

01

Design & complexity

Simple rectangular extensions with standard roof structures cost significantly less than projects involving complex geometries, structural steelwork, or bespoke glazing. Your architect's design choices have a direct impact on your build budget — which is why early-stage cost awareness matters.

02

Specification & finishes

The materials, fixtures, and finishes you choose can dramatically shift the cost per square metre. Aluminium bi-fold doors, underfloor heating, and bespoke joinery are common additions that add value to a project but also add meaningful cost. Clear specifications at tender stage keep your quotes accurate and comparable.

03

Location & site conditions

London commands higher labour rates than most of the UK. Beyond that, individual site conditions — sloping ground, restricted access, poor foundations, or proximity to neighbouring properties — can add cost that is impossible to predict without a proper site investigation.

Excela Architecture London — Home Extension Service

What Our Home Extension Architecture Service Includes

We offer a complete architectural service for home extension projects — from the initial measured survey through to planning permission. Our fixed fee gives you certainty from day one, with no hidden costs at the design stage.

£1,850

+ VAT  ·  fixed fee

• 3D laser measured building survey of the existing property

• Ordnance Survey location and block plans

• Existing information planning application drawings

• Concept design with unlimited revisions for 30 days

• Proposed planning application drawings

• Planning application submission and monitoring

Our Concept Design Stage — Unlimited Revisions for 30 Days

The concept design stage is where your extension comes to life. We begin by reviewing your project requirements and exploring the feasibility of your extension — considering structural opportunities, planning constraints, and how best to optimise the space and layout of your home.

We present our concept designs in full 3D as well as traditional floor plans and elevations. Exterior 3D visualisations are particularly valuable at this stage: they allow you to genuinely see how the extension will look and feel from outside, making design decisions far easier and more intuitive.

Throughout the concept design stage, you can make as many changes as you need. We do not charge per revision — our goal is to ensure you are completely satisfied with the final design before we progress to the planning application drawings.

Once you are 100% happy, we produce the proposed planning application drawings, submit the application on your behalf, and monitor it throughout the decision process — giving your application the best possible chance of approval.

Why Labour Costs Are the Biggest Variable

Labour accounts for approximately 40–45% of a total construction budget in the UK. In London, where skilled tradespeople command premium rates and competition for good contractors is high, this percentage can push even higher. This is one of the primary reasons why London extension costs consistently outpace the rest of the country.

It also means that the quality of your tender documentation has a direct financial impact. When your planning permission drawings, building regulations drawings, and specification documents are detailed and accurate, contractors can price your job with confidence. Vague documentation leads to wide variations between quotes — and often to unwelcome surprises once work is underway.

Working with independent home extension architects to manage your tender process — rather than leaving it entirely to a design-and-build contractor — gives you the clearest possible picture of your true costs before you commit.

Planning Permission and Your Extension Cost

Not all home extensions require planning permission. Many single-storey rear extensions in London fall within Permitted Development rights, meaning you can build without a formal application — subject to size limits and specific conditions. However, properties in conservation areas, listed buildings, or those that have already used their Permitted Development allowance will require a full planning application.

At Excela Architecture, our home extension service includes the planning application submission and monitoring as standard. We assess your project's permitted development status from the outset, so you know exactly what consents are required and what timeline to expect.

How to Get an Accurate Cost for Your Extension

The benchmark figures in this guide are a useful starting point — but they are not a substitute for a project-specific cost plan. The most reliable route to an accurate build cost for your home extension is:

Step 1 — Appoint your architect. Detailed drawings are the foundation of any accurate cost estimate. Without them, builders are pricing in uncertainty — and you will pay for it.

Step 2 — Progress to building regulations drawings. These detailed technical drawings allow your contractor to price every element of the build with confidence.

Step 3 — Run a competitive tender. Obtain at least three quotes from reputable contractors. With a complete set of drawings and specifications, those quotes should be meaningfully comparable.

Step 4 — Allow for contingency. Even on well-managed projects, unforeseen conditions can arise once groundwork begins. A 10% contingency on your build cost is a sensible minimum.

Home Extension Architects in London

Ready to Start Planning Your Extension?

Talk to our team at Excela Architecture London about your project. We will give you an honest assessment of what is feasible, what it is likely to cost, and how we can help you achieve it.