Rear Extension Cost in Clapham: A Realistic 2026 Breakdown

A single-storey rear extension is one of the most popular home improvements in Clapham. It extends your kitchen or living space into the garden, floods the ground floor with natural light, and — when done well — transforms how your home feels and functions. But before you start dreaming about floor-to-ceiling glazing and polished concrete floors, you need to understand what a rear extension actually costs in Clapham in 2026.

This guide gives you realistic rear extension cost figures for Clapham, broken down by size, specification, and all the extras that builders' quotes sometimes leave out. No surprises, no hidden costs — just honest numbers so you can plan properly.

Rear Extension Costs by Size

The depth of your rear extension is the single biggest factor in its cost. Here is what you can expect to pay for a single-storey rear extension on a typical Clapham terrace in 2026.

3-Metre Rear Extension

A 3-metre extension is the maximum depth you can build under permitted development rights without prior approval (on an attached house). On a Clapham terrace that is typically 4 to 5 metres wide at the rear, a 3-metre extension adds 12 to 15 square metres.

Cost range: £40,000 to £60,000 (building works only)

This is the most common size of rear extension in Clapham. It gives you enough extra depth for a decent kitchen-diner, a large seating area, or a family room that opens onto the garden.

4-Metre Rear Extension

A 4-metre extension requires prior approval under the larger home extension scheme (for attached houses extending beyond 3 metres but up to 6 metres). Prior approval is a simplified process — not a full planning application — but your neighbours are consulted.

Cost range: £50,000 to £75,000 (building works only)

The extra metre makes a significant difference to the usability of the space. A 4-metre extension on a 4.5-metre wide terrace gives you about 18 square metres — enough for a kitchen with island and a dining area.

6-Metre Rear Extension

The maximum permitted under the larger home extension scheme for an attached house. A 6-metre extension transforms the ground floor completely, but it does take a substantial chunk of your garden.

Cost range: £70,000 to £110,000 (building works only)

At this depth, you are creating a genuinely large room. On a typical Clapham terrace, a 6-metre extension adds 24 to 30 square metres — roughly doubling your ground floor living space.

Cost Per Square Metre

As a rule of thumb, rear extension costs in Clapham in 2026 run at approximately:

Standard specification: £2,200 - £2,800 per sqm
Mid-range specification: £2,800 - £3,500 per sqm
Premium specification: £3,500 - £4,500 per sqm

These per-square-metre figures include the building shell, roof, glazing, floor, electrics, plumbing, and basic finishes. They do not include the kitchen fit-out, furniture, or garden landscaping.

Use our renovation cost calculator for a personalised estimate based on your specific extension size and finish level.

What Is Included in a Rear Extension Quote

Understanding what is and is not included in a builder's quote is essential. Here is what a typical all-in quote for a rear extension in Clapham should cover.

Typically Included

Foundations: Excavation, concrete foundations, and ground floor slab
Walls: Blockwork or timber frame walls, insulation, internal plastering
Roof: Flat roof structure, insulation, membrane, and internal ceiling
Glazing: Bi-fold doors, sliding doors, or French doors to the garden
Structural steel: RSJ beams to open up the back wall of the existing house
Electrics: New circuits, lighting, sockets (first and second fix)
Plumbing: Waste connections, hot and cold water supply (first and second fix)
Floor: Screed or timber floor, ready for final floor covering
Drainage: Connection to the existing drainage system
Making good: Internal plastering where new meets old, decoration to new areas

Typically NOT Included (Budget Separately)

These are the items that catch people out. They are real costs that you will incur, but builders often exclude them from their base quote.

Kitchen fit-out: Units, worktops, appliances (£8,000 - £35,000 depending on spec)
Final flooring: Tiles, engineered timber, polished concrete (£1,500 - £5,000)
Underfloor heating: £1,500 - £3,500 for a rear extension
External landscaping: Patio, steps, garden restoration after the build (£2,000 - £8,000)
Skip hire and waste removal: £1,500 - £3,000 over the course of the project
Building regulations fees: £400 - £800
Structural engineer fees: £500 - £1,500
Architect/designer fees: £2,000 - £6,000 (typically 5-10% of build cost)
Party wall surveyor fees: £800 - £1,500 per neighbour
Rooflights: £600 - £1,500 each (if not included in the base specification)

When comparing quotes, make sure every builder is pricing the same scope of work. A quote that seems cheap may simply be leaving out items that a more expensive quote includes.

Foundations: The Hidden Cost

Foundations are typically the first significant unknown on any extension project. The depth and type of foundation depends on ground conditions, and in Clapham, two factors dominate.

London Clay

Most of Clapham sits on London Clay, which is a shrinkable soil. This means foundations must be deep enough to reach stable ground below the zone affected by seasonal moisture changes. Standard strip foundations on London Clay in Clapham are typically 1 to 1.5 metres deep.

Trees

Clapham is a leafy borough, and trees near your proposed extension can significantly affect foundation depth. Tree roots extract moisture from the clay, causing it to shrink and swell. If there are significant trees within influencing distance of your extension (roughly equal to the tree's mature height), your foundations may need to be 1.5 to 2.5 metres deep, or you may need piled foundations.

Piled foundations cost significantly more than standard strip foundations — typically £5,000 to £12,000 for a rear extension, compared to £3,000 to £5,000 for standard foundations. This is one of the biggest potential cost surprises.

Your structural engineer will advise on the appropriate foundation design based on a site survey and desktop study of ground conditions.

Structural Steel

Opening up the rear wall of your house to connect the new extension to the existing ground floor requires structural steel. The existing rear wall is load-bearing — it supports the first floor and everything above — so a steel beam (RSJ or UB) is needed to carry that load.

Typical Steel Costs

For a standard rear extension on a Clapham terrace:

Steel beam supply: £500 - £1,500 (depending on span and load)
Installation labour: £1,000 - £2,500 (temporary propping, lifting, positioning, grouting)
Structural engineer design: £400 - £800

Total steel costs are typically £2,000 to £5,000. If you are also removing the wall between front and rear reception rooms to go fully open plan, you will need a second beam — see our guide to open plan kitchens in Victorian terraces.

Glazing Options and Their Costs

The glazing at the rear of your extension is one of the most visible and impactful design decisions. It is also one of the biggest variables in cost.

Bi-Fold Doors

The most popular choice in Clapham. Bi-fold doors fold back in a concertina pattern to open up the rear wall.

Aluminium (standard): £3,000 - £5,000 for a 3 to 4 metre opening
Aluminium (slim profile, e.g., Schuco, Origin): £5,000 - £8,000
Timber: £4,000 - £7,000
Aluminium-clad timber (e.g., Rationel): £6,000 - £10,000

Sliding Doors

Increasingly popular as an alternative to bi-folds. Sliding doors have thinner frames and create a more minimalist look.

Standard sliding (2 or 3 panels): £3,500 - £6,000
Slim-frame sliding (e.g., IQ Glass, Vitrocsa): £6,000 - £15,000

Fixed Glazing With Pivot Doors

For a contemporary look, a combination of fixed glass panels with one or two pivot or hinged doors. This maximises the glass area and minimises frame lines.

Cost: £5,000 - £12,000 depending on size and specification

Rooflights

Rooflights in the flat roof bring light deep into the extension and the existing house beyond. They are particularly important if your extension is more than 3 metres deep, as the middle section can otherwise feel dark.

Fixed rooflights: £600 - £1,200 each
Opening rooflights: £800 - £1,500 each
Walk-on glass (for patios above): £2,000 - £5,000 per panel

Kitchen Fit-Out Within the Extension

If your rear extension is becoming a new kitchen (the most common scenario in Clapham), the kitchen fit-out is a separate cost on top of the building works.

For detailed kitchen costs, see our kitchen renovation cost breakdown for Clapham. In summary:

Standard kitchen: £8,000 - £15,000
Mid-range kitchen: £15,000 - £25,000
Premium/bespoke kitchen: £25,000 - £50,000+

These figures include units, worktops, sink, tap, and installation labour. Appliances are additional (£3,000 - £15,000 depending on brands).

VAT on Rear Extensions

All rear extension work on existing residential properties is subject to 20% VAT. This applies to both labour and materials. Make sure every quote you receive states clearly whether VAT is included.

For a £60,000 extension, VAT adds £12,000. This is a significant sum that must be factored into your budget from the start.

Some small builders and sole traders with turnover below the £90,000 VAT threshold are not VAT-registered and do not charge VAT. This can represent a meaningful saving, but ensure the builder has the capacity and experience for your project.

Professional Fees

Professional fees are a real cost that sits on top of the building works. Budget for:

Architect or architectural designer: £2,000 - £6,000 (or 5-10% of build cost for a full service)
Structural engineer: £500 - £1,500
Party wall surveyor: £800 - £1,500 per neighbour (see our guide on party wall obligations)
Building regulations application: £400 - £800
Planning application (if required): £258

Total professional fees for a typical Clapham rear extension: £4,000 to £10,000.

Timeline for a Rear Extension in Clapham

A realistic timeline from first design meeting to completed extension:

| Phase | Duration | |---|---| | Design and drawings | 4 - 8 weeks | | Planning permission (if needed) | 8 - 12 weeks | | Building regulations submission | 2 - 4 weeks | | Party wall notices | 8 - 10 weeks | | Contractor selection | 2 - 4 weeks | | Construction | 10 - 16 weeks | | Kitchen installation | 2 - 4 weeks |

Total: 6 to 12 months from start to finish, depending on whether you need planning permission and how quickly you can schedule a contractor.

Build a detailed schedule with our timeline estimator.

Is a Rear Extension Worth the Cost in Clapham?

Property values in Clapham make rear extensions a sound investment. A well-executed single-storey rear extension typically adds £40,000 to £80,000 to the value of a Clapham terrace, depending on the overall property and the quality of the work.

Given that the total project cost (extension plus kitchen plus professional fees) runs from £60,000 to £150,000, the financial return depends on the property value and how much you spend. The return is strongest on mid-value properties with a standard-to-premium specification — avoid over-capitalising with a specification that exceeds what the local market expects.

But the real value of a rear extension is not financial. It is the daily experience of a bright, spacious kitchen-diner that connects to the garden, and the end of the cramped, dark galley kitchen that defines so many un-renovated Clapham terraces.

For a personalised cost estimate and to understand the full scope of your project, start with our renovation cost calculator and our guide to choosing contractors in Clapham.

Rear Extension Costs in Clapham: A Realistic 2026 Breakdown