Extension costs in London range from £45,000 for a modest rear extension to well over £150,000 for a wraparound or lower-ground project. This guide breaks down 2026 build costs by extension type, explains what drives the London premium, and covers the hidden costs that most quotes leave out.
Costs vary significantly by borough, property type, soil conditions, and project complexity. The figures in this guide are based on London contractor rates in early 2026. They should be used for budgeting and feasibility — not as a substitute for contractor quotes.
All figures are build-only costs excluding VAT, architect fees, structural engineer, planning fees, and interior fit-out. They reflect mid-range London contractor rates for 2026. Budget and premium schemes will sit outside these ranges.
| Extension type | Low estimate | High estimate | Notes |
|---|---|---|---|
| Single-storey rear extension | £2,000 | £3,500 | Per m². Most common project type. |
| Side return extension | £2,500 | £4,000 | Per m². Common in Victorian terraces. |
| Wraparound (rear + side) | £3,000 | £4,500 | Per m². Structural complexity increases cost. |
| Two-storey rear extension | £2,200 | £3,800 | Per m². Cost per m² often lower than single-storey. |
| Lower-ground / basement | £3,500 | £6,000+ | Per m². Wide range depending on soil, waterproofing, and depth. |
| Loft conversion (dormer) | £50,000 | £80,000 | Total project cost, not per m². |
| Garage conversion | £15,000 | £35,000 | Total project cost. Lower if structure is sound. |
These costs are separate from the builder's quote and are frequently underestimated in initial budgets. For a typical London extension, professional fees and compliance costs add £8,000–£20,000 before interior fit-out.
Dependent on scope; full design and planning service sits toward the upper end. Many charge a % of build cost (typically 5–10%).
Householder application fee in England from April 2025 (increased from £258). Pre-application advice costs extra — typically £200–£600 depending on borough.
Required for most extensions involving removal of load-bearing walls, steel beams, or significant foundation work.
Required if the work is within 3m of a neighbour's foundation or involves a shared wall. Each neighbour can appoint their own surveyor, with costs typically borne by the building owner.
Charged by the local authority (or an approved inspector) to inspect the works at key stages. Required for all extensions regardless of planning status.
Kitchen, bathroom, flooring, and lighting are usually excluded from builder quotes. A new kitchen alone can add £8,000–£25,000.
Underpinning or unexpected foundation depth — common on London clay soils where foundations can be shallower than expected
Drainage diversions — an existing drain under the extension footprint can add £2,000–£8,000 to the project
Planning delays — if your application takes longer than expected, your builder may move to another project, leading to re-mobilisation costs
Asbestos removal — present in many London homes built before 1985, particularly in textured ceilings, floor tiles, and roof felt
Temporary accommodation — if the works make the kitchen or bathroom unusable for several weeks, short-term rental costs can be significant
VAT — most builder quotes exclude VAT. At 20% on a £60,000 build, this adds £12,000
A 25m² single-storey rear extension on a Victorian terrace in London — including opening up to the kitchen, new lantern roof light, and bi-fold doors:
This is an illustrative mid-range estimate. Premium finishes, complex site conditions, or a borough with higher professional fee expectations (e.g. Camden, Westminster) will push costs higher. A budget scheme with simpler finishes and a flat roof could come in at £70,000–£80,000 all-in.
A 25m² single-storey rear extension typically costs £50,000–£87,500 in build costs (at £2,000–£3,500/m²). Add architect fees (£2,000–£6,000), structural engineer (£800–£2,000), planning application (£528), building regulations, party wall, and interior fit-out, and a realistic all-in budget before VAT is £75,000–£110,000. With VAT at 20%, expect £90,000–£130,000 total.
Yes. Extensions approved under Building Regulations from 2024 onwards must meet improved energy efficiency standards under Part L. This typically requires better-specified insulation, higher-performance glazing, and in some cases low-carbon heating integration. The cost premium on the fabric and services elements of the build is typically 5–15%.
Labour costs in London are typically 20–40% higher than the national average. Material delivery and site logistics in dense urban areas add cost. Planning complexity — Conservation Areas, party wall obligations, and borough-specific design requirements — increases professional fees. London soil conditions (particularly London clay) also add foundation complexity that builders outside the capital rarely encounter.
Absolutely — and it should be the first step, not the last. If your property is in a Conservation Area, covered by an Article 4 Direction, or in a borough that is likely to require significant design revisions, your architect and application costs will be higher. Knowing this before commissioning drawings avoids wasted spend on schemes that face a structural planning obstacle.
The most expensive planning mistake in London is commissioning an architect before checking whether the project is feasible at all. Use our tool to check your address-level constraints, see nearby precedents, and understand whether you need Permitted Development or a full application — in seconds, before any fee is paid.