Double Storey Extension Cost London 2026: Prices, Planning and What to Expect

A double storey extension adds space on both the ground and first floor of your London home, making it the most cost-effective way to significantly increase your living space. Rather than building a single storey and a separate first floor addition separately, a double storey extension achieves both in one build — at considerably less cost per square metre than a single storey alone. In 2026, a double storey extension in London typically costs between £70,000 and £150,000 depending on size and specification.

A completed double storey extension in London by LCCL Construction

LCCL Construction has delivered hundreds of double storey extensions across London and Hertfordshire. This guide covers double storey extension costs, what affects the price, planning permission requirements and everything you need to know before starting your project.

How Much Does a Double Storey Extension Cost in London in 2026?

The cost of a double storey extension in London depends on the footprint size, specification and your location. Below is a breakdown of typical estimated costs in 2026:

Standard double storey extension (20–30 sqm total): £70,000 to £90,000. A double storey rear extension of 3 metres depth on both ground and first floor. Ground floor typically provides kitchen-diner space; first floor adds a new bedroom and bathroom.

Mid-range double storey extension (30–45 sqm total): £90,000 to £120,000. The most common range for a well-specified double storey extension in North London. Includes good quality glazing, a full bathroom or en-suite on the first floor and quality internal finishes.

High-specification double storey extension (45 sqm+): £120,000 to £150,000+. Premium specification throughout, full en-suite bathroom, high-end kitchen, bifold doors, large-format glazing. Common in higher value areas such as Hampstead, Muswell Hill, Highgate and Islington.

Why Is a Double Storey Extension Better Value Than a Single Storey?

The foundations, structural frame and roof of a double storey extension cost only marginally more than those of a single storey extension with the same footprint. Because these elements are the most expensive part of any extension, adding a second storey above them is the most cost-effective way to double the floor space gained. In practical terms, a double storey extension typically costs 30–40% more than a single storey on the same footprint, but delivers twice the floor area.

What Affects the Cost of a Double Storey Extension in London?

Footprint size: The larger the ground floor footprint, the higher the cost. Most double storey extensions in London are 3–6 metres deep and the full width of the property or a substantial portion of it.

Bathroom specification: The first floor addition typically includes a new bedroom and bathroom or en-suite. Bathroom specification varies widely — from a standard suite at £4,000–6,000 to a premium en-suite at £10,000–15,000.

Structural complexity: Opening up the back wall of the existing house on both ground and first floor levels requires structural steelwork. This is standard on most double storey projects and is included in our cost estimates.

Location: Labour and material costs vary across London. LCCL Construction provides detailed cost estimates for every area we cover across London and Hertfordshire.

Do You Need Planning Permission for a Double Storey Extension in London?

Double storey rear extensions can sometimes be built under permitted development rights, but the rules are more restrictive than for single storey extensions. The extension must not extend beyond 3 metres from the rear wall, must not exceed the height of the existing roof, and must not be within 7 metres of the rear boundary. In practice, many double storey extensions in London require a planning application.

Properties in conservation areas, listed buildings and flats always require planning permission. LCCL Construction assesses planning requirements for every project as part of our free initial consultation and handles all planning applications as part of our design and build service.

How Long Does a Double Storey Extension Take to Build?

A double storey extension typically takes 16 to 24 weeks to build from the day work starts on site. The full project timeline from consultation to completion is usually 8 to 14 months, depending on whether planning permission is required and the complexity of the project.

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

A double storey extension typically adds between £100,000 and £200,000 to a London property value, significantly exceeding the cost of the project in most cases. Adding a bedroom and bathroom on the first floor while simultaneously creating a larger ground floor living space delivers the highest return on investment of any single extension project. For more detail see our guide on how much value a house extension adds to a London property.

Why Choose LCCL Construction for Your Double Storey Extension?

LCCL Construction is a specialist house extension builder in London with over 20 years of experience, 200+ verified reviews at 4.7/5 and LABC Award Winners 2024. As a design and build company, we handle every stage from architectural drawings through to completion under one roof, with a single project manager throughout.

To get a free consultation and detailed cost estimate, call 020 8037 7332 or book online to arrange a free site visit.