Double Storey Extension Cost London: 2026 Price Guide

A double storey extension is one of the most impactful changes you can make to a London home. By extending on both ground and first floor simultaneously, you can add a large open-plan kitchen-diner below and one or two bedrooms above — transforming a cramped family home into something that genuinely works for modern life. But the costs are substantial, and almost every London double storey project requires planning permission. Here's everything you need to know in 2026.

Double Storey Extension Costs in London in 2026

Cost per square metre

A double storey extension in London costs between £2,800 and £4,500 per square metre for the combined ground and first floor build. The per-square-metre cost is generally lower than a single storey extension because the foundations, scaffolding, and roof are shared across two floors — making it one of the most cost-efficient ways to add large amounts of space.

Typical total project costs

• Standard (20–30m² total): £120,000–£170,000

• Mid-range (30–45m² total): £150,000–£220,000

• Large or high-spec: £200,000–£300,000+

These are all-in figures including construction, professional fees, planning, and VAT. Kitchen and bathroom fit-out are additional.

Why Double Storey Extensions Cost More Than You Expect

Structural requirements across two floors

A double storey extension isn't simply twice the cost of a single storey — it's structurally more complex. The ground floor structure must support the first floor, which means heavier foundations, structural steelwork to carry upper floor loads, and more detailed structural engineering. These costs are significant and not always well communicated in early-stage estimates.

London's labour and logistics premium

As with all London extension work, expect to pay 25–40% more than you would for an equivalent project elsewhere in the UK. Skilled trades are in high demand, access to terraced properties is often challenging, and material delivery costs in central and inner London are higher.

Higher professional fees

Almost all double storey extensions require full planning permission, which means more detailed architectural drawings, potentially longer planning timescales, and additional consultants (structural engineer, sometimes a planning consultant) all add to the professional fee total. Budget 15–20% of build cost for all professional services.

Full Cost Breakdown

• Construction: £90,000–£175,000 (shell only, excluding fit-out)

• Architect / design and planning consultant: £7,000–£18,000

• Planning application fee: £548 (from April 2026)

• Structural engineer: £2,500–£5,000

• Building regulations: £1,000–£2,000

• Party wall surveyor: £800–£2,000 per adjoining neighbour

• Kitchen fit-out: £15,000–£50,000+ (additional)

• Bathroom(s): £8,000–£20,000 each (additional)

Planning Permission for Double Storey Extensions in London

When permitted development applies

Permitted development rights do allow some two-storey rear extensions, but the rules are strict: the extension cannot project more than 3 metres beyond the original rear wall, must not exceed the height of the existing roof, and the upper floor windows in side elevations must be obscure-glazed and fixed shut. In practice, very few double storey extensions in London qualify — most are either too deep, affect the roofline, or are in conservation areas where PD rights are restricted.

When you need a full planning application

If your proposed extension exceeds the permitted development limits — which most worthwhile double storey projects do — you'll need a householder planning application. Your council will assess the design against their local planning policies, with particular attention to impact on neighbouring properties' light and privacy, and the visual impact on the street scene.

Conservation areas and Article 4 Directions

If your home is in a conservation area, double storey extensions almost always require planning permission regardless of size. The council's conservation officer will scrutinise materials, proportions, and the extension's relationship to the existing building more carefully than they would in a standard residential area.

How Much Value Does a Double Storey Extension Add?

A well-designed double storey extension is one of the highest-return investments available to London homeowners. Adding a large kitchen-diner on the ground floor and a fourth bedroom plus bathroom above can add 15–25% to a property's value — often exceeding the cost of the work in London's family home market. The addition of a fourth bedroom is particularly valuable, as it moves the property into a completely different buyer category.

Is Double Storey Worth the Extra Cost Over Single Storey?

Space gained per pound spent

A double storey extension typically adds twice the floor area of an equivalent single storey for roughly 50–60% more cost. That makes it considerably better value per square metre — particularly important when London construction costs and professional fees are as high as they are.

When double storey is the smarter choice

Double storey is the right choice when you need both more ground floor living space and additional bedrooms or bathrooms. If you only need more kitchen or living space, a single storey is simpler and less likely to face planning objections. If you know you'll eventually want both, doing them simultaneously in one project avoids double disruption, double professional fees, and the higher cost of mobilising contractors twice.

The Full Project Timeline: Design to Completion

• Feasibility study and initial design: 4–6 weeks

• Planning application preparation: 4–6 weeks

• Planning determination: 8–13 weeks

• Building regulations drawings: 4–8 weeks (can run in parallel)

• Contractor procurement: 4–8 weeks

• Construction on site: 16–24 weeks

• Total from first call to completion: typically 10–16 months

How to Choose the Right Design and Planning Team

For a project of this scale, professional guidance is not optional — it's essential. A good architectural design and planning consultant will optimise the design to maximise space and natural light, manage the planning process to avoid delays and refusals, produce detailed drawings that give contractors clear scope to price against, and help you navigate building regulations and party wall obligations. At Studio : MASS, we manage every stage from feasibility through to build-ready drawings, with over 20 years of experience on London residential projects.

Frequently Asked Questions

Does a double storey extension always need planning permission?

In most cases in London, yes. While there are limited circumstances where a two-storey rear extension can be built under permitted development, the restrictions are strict and most worthwhile projects exceed them. Properties in conservation areas always require planning permission for a double storey extension.

How long does a double storey extension take from start to finish?

A typical London double storey project takes 10–16 months from first appointment to practical completion. Construction alone takes 16–24 weeks on site.

Will my neighbours object to a double storey extension?

Neighbours have the right to comment on planning applications, and double storey extensions are more likely to generate objections than single storey ones due to their potential impact on light and privacy. A good planning consultant will design the scheme to minimise these impacts and prepare a strong application that anticipates and addresses likely concerns.

Ready to explore whether a double storey extension is right for your home? Book a free architectural advice call with Studio : MASS. We'll give you an honest assessment of feasibility, planning prospects, and realistic costs — with no obligation.