House Extension Costs in South West London: What a Quote Should Include

A practical ex-VAT extension cost guide for rear, side-return and wraparound projects, plus the questions that make contractor quotes comparable.

The useful question is not simply “how much is an extension?” It is “what exactly is included for this property, design and finish?” Two quotes can use the same floor area and still cover very different work.

Current build-cost guides

Bluebird's published area-page guide uses these starting ranges:

Side-return extension: £40,000–£60,000 ex VAT
Rear extension: £60,000–£90,000 ex VAT

Larger wraparound schemes, complex structure, restricted access and premium glazing or finishes can cost more. Use the cost calculator for an early build-budget guide. It excludes VAT, professional fees, planning and building-control fees, surveys and client-supplied fixtures unless a written quote states otherwise.

Six items that change the price

1. Structure and openings

Removing more of the existing rear wall can require larger steels, temporary works and padstones. The final design should be coordinated with the structural engineer before a contractor promises a fixed figure.

2. Ground conditions and drainage

Foundations, trees, neighbouring structures and drains can affect both design and sequence. Existing drains may need protection or diversion, and a build-over agreement may be relevant where a public sewer is involved.

3. Access and site logistics

Many South West London terraces have no direct rear access. Waste, concrete, steels and glazing may need to travel through or over the house. Parking suspensions and restricted working hours can also affect the programme.

4. Glazing and external envelope

Large sliding doors, rooflights and structural glazing can be a major part of the budget and have long lead times. The quote should name the system or include a clear allowance.

5. Kitchen and finishes

A shell extension and a completed kitchen-diner are not the same product. Confirm whether the quote includes the kitchen, appliances, worktops, floor finishes, decorating and final lighting.

6. Work to the existing house

Heating upgrades, rewiring, plaster repairs and decoration outside the new footprint can add cost. Ask the contractor to identify those items rather than hiding them inside a broad allowance.

Make quotes comparable

Request an itemised proposal with:

the drawing revision and specification used for pricing;
a clear list of inclusions, exclusions and provisional allowances;
VAT shown consistently;
the programme and payment schedule;
responsibility for design coordination and approvals;
how variations will be priced and agreed;
site protection, waste and making good.

Completed project evidence is more useful than a generic gallery. See the Wandsworth side-return extension, Earlsfield side-return extension, Clapham double-storey extension and Sheen rear extension.

Check planning before treating the design as fixed

Some house extensions may use permitted development, while others need full planning permission or prior approval. Flats and maisonettes are different. Conservation areas, Article 4 directions, listed status, earlier additions and council policy all matter.

Use the relevant local page for an address-specific starting point:

The right time to compare final quotes is after the main design, structural assumptions and finish level are clear. Before that, treat numbers as budget guides and ask what evidence supports them.