London House Extension Cost – The Essential 2025 Guide

Extending a house in London is one of the most effective ways to add space and value — but costs can vary widely depending on structure, access, logistics and design. This guide gives you realistic cost expectations, explains the key price drivers, and helps you budget correctly before committing to architects or builders.

1. Typical Cost Ranges for House Extensions in London

Below are the current estimated cost bands for different types of extensions in London:

Single-storey rear or side extension:
£1,800 – £2,800 per m²
Ideal for kitchen/diner expansions, open-plan living and improving ground-floor layouts.

Wrap-around or larger footprint extensions:
£2,200 – £3,200 per m²
Includes more complex layouts, steelwork and higher material volumes.

Double-storey extensions:
£2,800 – £4,200 per m²
Provides extra bedrooms/bathrooms; higher structural and compliance requirements.

Loft conversions:
£1,500 – £2,200 per m² (converted space)
Includes dormer construction, insulation, electrics and finishes.

Typical example:
A 25 m² single-storey extension often falls between £45,000–£70,000, excluding kitchen/bathroom fixtures and premium finishes.

2. What Drives the Cost of a London House Extension

Extension prices vary based on a combination of factors:

Structural complexity

Steel beams, foundation depth, soil conditions, drainage alterations and load-bearing changes.

Design and layout

Wrap-around shapes, open-plan layouts, multiple floors or bespoke architectural detailing.

Access and London logistics

Narrow back passages, no side access, parking restrictions, scaffolding requirements, skip permits.

Planning requirements

Permitted development vs planning permission, conservation areas, heritage restrictions, party wall agreements.

Finishes and specification

Flooring, glazing, doors, insulation, roof lights, MVHR, UFH, bespoke joinery, premium materials.

External works & landscaping

Patios, decking, steps, retaining walls, drainage upgrades.

Compliance and overheads

Project management, waste disposal, CDM compliance, health & safety, insurance and contractor overhead.

3. How to Budget Properly Before You Start Design Work

A realistic budget makes your project smoother and avoids unexpected cost shocks.

Step 1 – Calculate approximate floor area
Multiply the square metres you plan to add.

Step 2 – Apply the cost-per-m² ranges
Use the figures in Section 1 to form a realistic band (low to high).

Step 3 – Add a 10–15% contingency
Covers unforeseen structural issues or specification changes.

Step 4 – Factor in professional fees
Architects, structural engineers, building control, planning consultants, party-wall surveyors.

Step 5 – Include fixtures and finishes
Kitchens, bathrooms, flooring, lighting and decoration are typically outside the base build cost.

Having this early estimate helps you decide whether extending or upsizing is the better long-term choice.

4. How to Reduce the Cost of Your Extension (Without Sacrificing Quality)

Keep the shape simple

Rectangular extensions require fewer beams and structural changes.

Avoid unnecessary reconfiguration

Moving staircases, drainage or load-bearing walls can dramatically increase costs.

Choose cost-effective materials

Use standard sizes and modular elements instead of full bespoke joinery.

Use permitted development where possible

Saves planning fees, reduces delays and simplifies design.

Combine works into one phase

More efficient than multiple small projects.

Work with a design-and-build contractor

A single team reduces communication gaps, redesign fees and scope misunderstanding.

5. Why Homeowners Trust KT5 Construction with Extension Costs

• 25+ years of London & Surrey build experience
  We know what realistic budgets look like and how to avoid unnecessary cost escalation.

• Transparent, itemised quotations
  Every stage of the build is broken down clearly with no hidden extras.

• Fixed-price or guaranteed maximum price options
  Gives confidence and prevents mid-project surprises.

• Professional project management
  Coordination of trades, logistics and compliance tailored for London properties.

• Strong track record
  View our portfolio for recent extensions, kitchens, refurbishments and design-build projects.

6. Next Steps: Get Your Accurate Extension Cost

You can take the next step in two ways:

Get an instant ballpark figure

Use our Free Extension Cost Calculator:
https://www.kt5construction.com/free-extension-cost-calculator/

Request a detailed fixed-price quote

Send us your drawings, plans, photos or a Rightmove link and we’ll prepare a clear, itemised cost breakdown with assumptions, specification levels and timeline.

Ready to plan your extension?
Contact KT5 Construction to discuss your project.