Double storey extension in London: complete guide

What is a double storey extension?

A double storey extension -- also called a two-storey extension -- adds new living space on both the ground floor and first floor simultaneously. The most common configuration is a kitchen or kitchen-diner on the ground floor with a bedroom or bathroom on the first floor above. This gives you roughly twice the floor area of a single-storey extension for only around 50-60% more cost, because you share the same foundations, roof, and much of the structural work.

In London, where land values are high and plot sizes constrained, a double storey extension is one of the most cost-effective ways to add significant space to a family home. A typical 3-metre deep, full-width two-storey rear extension on a terraced house adds 25-35 sqm of new floor space -- enough for a generous kitchen-diner below and a master bedroom with en-suite above.

Double storey extensions can be built at the rear, the side, or as a combination of both (a two-storey wraparound). Each configuration has different planning implications and design challenges, which we cover below.

Planning permission: always required

Unlike single-storey rear extensions, which can often proceed under Permitted Development, double storey extensions always require planning permission. There are no PD rights for two-storey extensions under any circumstances. The application is a standard householder planning application (£258, 8-week determination period).

What planning officers assess

• Scale and massing: The extension should appear subordinate to the original house. A two-storey extension that is as wide and as deep as the existing house will likely be refused as overdevelopment.
• Impact on neighbours: The 45-degree rule is commonly applied -- an imaginary line drawn at 45 degrees from the centre of your neighbour's nearest habitable window should not be breached. Loss of light, overshadowing, and overbearing impact are assessed.
• Overlooking and privacy: First-floor windows in the extension must not directly overlook neighbouring gardens or habitable rooms. The standard minimum back-to-back distance is 21 metres between facing habitable windows.
• Materials and design: The extension must use matching materials -- brickwork that matches the existing house in colour, bond, and mortar, and roof tiles or slates that match the existing roof. Planning officers regularly refuse extensions with materials that do not match.
• Roof design: A pitched roof that matches the existing roof angle is usually expected. Flat-roofed two-storey extensions are more contentious and are often refused on design grounds unless there is a specific design justification.

We strongly recommend pre-application advice for double storey extensions. At £300-£600 depending on the borough, it gives you written officer feedback before you commit to the full application fee and professional costs. We prepare the pre-application drawings and handle the submission.

The 3-metre rear projection guideline

Most London boroughs apply an informal guideline of 3 metres maximum rear projection for two-storey extensions on terraced and semi-detached houses. This is not a hard rule -- it is a starting point for assessment. On detached houses, or where the site has unusual characteristics (e.g., no adjoining neighbour on one side), a deeper projection may be acceptable. Conversely, in tight terraces with short gardens, even 3 metres may be resisted if the impact on neighbours is significant.

Structural and foundation requirements

A double storey extension carries significantly more load than a single storey, which means the structural design is more demanding.

Foundations

Foundations for a two-storey extension must support twice the wall height and the additional floor loads. In London clay, strip foundations are typically 1-1.2 metres deep minimum, increasing to 2 metres or more near mature trees (due to clay shrinkage). Where trees are close or the soil conditions are poor, piled foundations may be required, adding £5,000-£15,000 to the cost. A desktop investigation of the soil and tree conditions is essential before finalising the structural design.

Structural openings

Where the extension connects to the existing house, existing walls are typically removed or opened up with steel beams. For a two-storey extension, steels at both ground and first floor level are common. The ground-floor steel supports the first-floor structure above the opening; the first-floor steel supports any roof structure above. The structural engineer designs the beam sizes, bearing details, and any required temporary propping during construction.

First floor structure

The new first floor is usually formed with timber joists or engineered timber I-joists spanning between the new external walls and the existing house wall. For spans over 4 metres, steel secondary beams may be needed. The floor must comply with Part E sound insulation standards if there is a habitable room below.

Matching materials: the make-or-break detail

Matching materials is the single most important design consideration for a two-storey extension. Because the extension is the same height as the existing house and sits directly alongside it, any mismatch in brick colour, bond pattern, mortar colour, or roof tiles is immediately and permanently visible.

• Bricks: Source matching bricks from a specialist reclamation yard or brick matching service. For London stock brick (the yellow/brown brick common on Victorian houses), services like Brick Warehouse, Imperial Bricks, or London Brick Company can supply close matches. Allow £1.20-£2.50 per brick for good reclaimed London stocks.
• Mortar: Match the mortar colour, joint profile (flush, weatherstruck, or bucket-handle), and joint width. Sample panels should be built and approved before the main brickwork begins.
• Roof: Use reclaimed tiles or slates that match the existing roof. Where the extension roof meets the existing roof, the junction must be weathered with matching lead flashings.
• Windows: Match the existing window style, proportions, and material. In conservation areas, timber windows may be required even if the existing house has uPVC replacements (the original windows are the reference point, not the replacement ones).

Double storey extension costs in London

Total project cost for a 30 sqm double storey extension in London: £60,000-£110,000 including all professional fees, council fees, and building works. The cost per sqm is significantly lower than a single-storey extension because the foundations, roof, and much of the structural work are shared across two floors.

Our Complete drawing package from £1,750 includes the measured survey, existing and proposed floor plans and elevations, section drawings, planning application submission, and building regulations drawings. Get a free quote.

Build timeline

A double storey extension is a substantial construction project. Here is a realistic timeline for a London project:

1. Design and planning (12-16 weeks): Measured survey, design development, pre-application advice (optional), planning application submission and determination
2. Building regulations and structural (4-6 weeks): Building regs application, structural engineer calculations and detailing
3. Party Wall (2-4 months): Serving notices and obtaining agreements (can run in parallel with planning)
4. Tendering and contractor appointment (2-4 weeks): Getting quotes, reviewing, and appointing a builder
5. Construction (14-20 weeks): Foundations, walls, first floor, roof, internal fit-out, services, decoration

Total timeline: 8-12 months from initial instruction to moving into the completed space.

Frequently asked questions