Rear Extension Cost London 2026: Complete Price Guide by Size and Specification

A rear extension is the most popular type of home extension in London — it adds a large, light-filled ground floor space at the back of the property, typically creating an open plan kitchen and dining area. This guide gives you accurate 2026 rear extension costs across all size ranges and specifications so you can budget realistically before starting your project. LCCL Construction has delivered hundreds of rear extensions across London and Hertfordshire as part of our complete design and build service.

LCCL Construction — specialist rear extension builders across London

How Much Does a Rear Extension Cost in London in 2026?

A single storey rear extension in London costs £35,000 to £75,000 in 2026 for a full design and build service.

A double storey rear extension costs £75,000 to £145,000.

This includes all architectural design, planning permission, building regulations, structural calculations, party wall notices where required and all construction works from foundations to finished plaster.

Single Storey Rear Extension Cost by Size

Small single storey rear (12 sqm): £35,000 to £48,000.

Standard single storey rear (18 sqm): £48,000 to £62,000.

Large single storey rear (25 sqm): £62,000 to £75,000.

Premium specification with bifold doors and lantern rooflight in Central London can exceed £85,000.

Double Storey Rear Extension Cost by Size

Small double storey rear (24 sqm total): £75,000 to £98,000.

Standard double storey rear (32 sqm total): £98,000 to £125,000.

Large double storey rear (44 sqm total): £125,000 to £145,000.

See our full double storey extension guide.

What Is Included in a Rear Extension Cost?

LCCL Construction rear extension costs include everything: measured building survey, architectural design, planning permission application where required, building regulations, structural engineer calculations, party wall notices and agreements where required, groundworks and foundations, structural steelwork including RSJ beams to remove the rear wall, superstructure, roof, external walls with matching brickwork or render, windows and doors including bifolds or sliding doors, first fix electrics and plumbing, insulation to current building regs standards, plastering, second fix and decoration.

What Affects Rear Extension Cost in London?

The main factors are: size in square metres, single or double storey, specification of glazing including bifold doors sliding doors and rooflights, complexity of steelwork to open up the existing rear wall, planning permission requirements, whether a party wall agreement is needed, location within London and quality of internal finishes. Premium London postcodes typically cost 15-20 per cent more than outer London.

Rear Extension Cost Per Square Metre in London

Rear extension costs in London typically range from £2,400 to £3,600 per square metre for a single storey and £2,600 to £3,800 per square metre for a double storey. Extensions are subject to 20 per cent VAT — unlike new build construction which is zero rated.

Rear Extension vs Side Return vs Wraparound: Cost Comparison

Rear only costs £35,000 to £75,000.

Side return only costs £35,000 to £75,000.

Wraparound combining both costs £60,000 to £128,000.

For Victorian terraces with a side alley a wraparound is often the highest value option. See our full home extension cost guide.

How Much Value Does a Rear Extension Add?

A well-designed rear extension in London typically adds 150 to 250 per cent of the construction cost in property value.

A £55,000 single storey rear extension typically adds £85,000 to £140,000 in value.

A £110,000 double storey rear extension typically adds £180,000 to £275,000.

See our full guide on how much value an extension adds.

Frequently Asked Questions About Rear Extension Cost in London

How much does a rear extension cost in London?

From £35,000 for a small single storey to £145,000 for a large double storey with premium specification.

How long does a rear extension take?

Typically 12 to 16 weeks on site for a single storey and 18 to 24 weeks for a double storey.

To get a free detailed estimate call LCCL Construction on 020 8037 7332 or get an instant quote online.