Double Storey Extension London: The Complete 2026 Guide

A double storey extension is one of the most transformative changes you can make to a London home. Unlike a single storey, which adds one large room at ground level, a double storey adds meaningful space on both floors — typically a new kitchen or living space below, and one or two bedrooms above. It's also one of the best value-per-square-metre additions you can make. This guide covers everything you need to know about planning, designing, and building a double storey extension in London.

Is a Double Storey Extension Right for Your London Home?

When double storey makes more sense than two separate projects

If you know you'll eventually want both more ground floor living space and additional bedrooms, doing both in a single double storey project is significantly more cost-effective than doing two separate projects over time. You pay for foundations, scaffolding, and professional fees once rather than twice. You endure disruption once. And the per-square-metre cost of the double storey is lower than either project done individually.

Suitable property types in London

Double storey rear extensions work on virtually all house types — terraced, semi-detached, and detached. Mid-terraced houses benefit particularly from the ability to increase footprint without side extensions. Properties where the ground floor kitchen and first floor bedroom provision are both inadequate for the family's needs are ideal candidates.

Structural and site considerations

The primary structural consideration is whether the existing party walls and foundations can accommodate the additional load of a first floor. Your structural engineer will assess this as part of the design process. On most standard London terraced properties, this is manageable. More complex sites — those with poor ground conditions, proximity to trees, or unusual foundation types — may require more significant structural interventions.

What Can You Add with a Double Storey Extension?

Ground floor possibilities

The most popular ground floor addition is an open-plan kitchen-diner with direct access to the garden — the archetypal London family home improvement. Large format glazing, bi-fold or sliding doors, roof lanterns, and kitchen islands all feature prominently in double storey projects. You can also use the ground floor for a utility room, playroom, or additional reception space, depending on your priorities.

First floor possibilities

The first floor addition most commonly provides an additional bedroom — often with an en-suite — or a family bathroom upgrade. Some homeowners use the first floor addition for a home office, particularly since remote working has become more prevalent. The combination of a master bedroom with en-suite above a new kitchen below is the most common configuration for London double storey projects.

Open plan design and connection to the garden

One of the key design opportunities in a double storey rear extension is the relationship between the new ground floor space and the rear garden. Floor-to-ceiling glazing, sliding or bi-fold doors, and careful consideration of levels (particularly where the garden is at a different level to the ground floor) can create a genuinely seamless indoor-outdoor connection that transforms how the house feels.

Planning Permission for Double Storey Extensions in London

When permitted development applies

A limited class of two-storey rear extensions can be built under permitted development: the extension must project no more than 3 metres beyond the original rear wall, must not be closer than 7 metres to the rear boundary, must not exceed the height of the existing roof, and upper floor windows in side elevations must be obscure-glazed and non-opening below 1.7 metres. In practice, very few double storey extensions in London meet all these criteria — most are deeper than 3 metres, or are in conservation areas where PD is removed.

When you need a full planning application

Most double storey extensions in London require a householder planning application. Your council will assess the proposal against its planning policies, with particular attention to impact on neighbouring properties — light, overlooking, and the physical relationship between the new building and adjacent homes.

Conservation areas and listed buildings

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 design's relationship to the existing building. Listed buildings require Listed Building Consent in addition to planning permission for any external alteration.

Neighbour considerations: the 45-degree rule

London planning authorities routinely apply the 45-degree test (or BRE daylight test) to assess the impact of a double storey extension on neighbouring properties' light. The test checks whether the proposed extension falls within a 45-degree line drawn from the centre of the nearest habitable room window of neighbouring properties. If it does, the council may require the extension to be reduced in size or modified in design. A good architect will design around this from the outset to avoid the need for redesign after submission.

How Much Does a Double Storey Extension Cost in London?

Cost per m² in 2026

Double storey extensions cost between £2,800 and £4,500 per square metre for the combined build. The lower per-square-metre cost compared to single storey extensions (where the same range runs higher) reflects the efficiency of shared foundations, roofing, and scaffolding across two floors.

Total project cost ranges

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

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

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

Professional fees

Budget 15–20% of build cost for professional services: architect and planning consultant, structural engineer, planning application (£548 from April 2026), building regulations, and party wall surveyor where required.

Design Principles for Double Storey Extensions in London

Matching materials to your existing house

The most successful double storey extensions read as a coherent part of the existing building rather than an afterthought. This usually means matching the brickwork, roof tile material, and window profile of the original house. In conservation areas, this is a planning requirement. Even outside conservation areas, a well-matched extension will look better and be easier to get approved.

Roof form and planning

The roof design of a double storey extension is a significant planning consideration. A flat roof is practical and cost-effective but may be refused in some contexts. A pitched roof that matches the existing house is usually the safest design choice for planning purposes but adds cost and complexity. Your architect will advise on the right approach for your specific property and council.

Internal layout and floor-to-floor flow

The staircase is often the most challenging design element in a double storey extension — particularly on terraced properties where the existing staircase layout may conflict with the new first floor plan. Getting the internal layout right requires careful coordination between the ground and first floor plans from the earliest stage of design.

How Much Value Does a Double Storey Extension Add in London?

A well-designed double storey extension typically adds 15–25% to a London property's value — often significantly more than the cost of the project. Adding a fourth bedroom to a 3-bed property moves it into a different buyer category entirely; adding a third bedroom to a 2-bed does the same. The return on investment is typically highest in London's family home market, where bedroom count drives pricing disproportionately.

The Full Project Timeline

• 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: 10–16 months

Party Wall Act: What You Need to Know Before Work Starts

Almost every double storey rear extension on a London terraced property triggers the Party Wall etc. Act 1996. You'll need to serve notice on any neighbour whose property is adjacent to or below the proposed works. For a typical terraced house double storey, this usually means both adjoining neighbours. The notice period is two months for most party wall works. Failure to serve notice before starting work is a common and costly mistake — it can result in injunctions stopping the build.

Common Mistakes That Cause Delays and Refusals

• Not checking whether permitted development applies before spending money on a full planning application

• Designing beyond the 45-degree daylight test line, resulting in a mandatory redesign post-submission

• Failing to serve party wall notices before starting work

• Underestimating structural complexity and costs for first-floor load transfer

• Not accounting for the fire safety requirements for the new staircase configuration

• Choosing inappropriate materials that clash with the existing house or fail conservation area policy

Frequently Asked Questions

Do I need planning permission for a double storey extension?

In most cases in London, yes. While a narrow category of two-storey rear extensions falls under permitted development, the rules are strict and most worthwhile double storey projects exceed them. Conservation area properties always require planning permission.

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

Typically 10–16 months from appointing your design consultant to practical completion. The build itself takes 16–24 weeks on site.

Will my neighbours object?

Double storey extensions are more likely to generate neighbour objections than single storey ones, primarily due to concerns about light, overlooking, and the visual impact from neighbouring gardens. A well-designed scheme that addresses these concerns upfront — and a planning consultant who prepares a strong application — significantly reduces this risk.

A double storey extension is a major project that rewards proper professional guidance from the start. Book a free architectural advice call with Studio : MASS and let's explore what's achievable for your home — including realistic costs, planning prospects, and a programme that works for you.