How Much Does a House Extension Cost in London? (2026 Guide)
A house-extension budgeting guide covering type, size, specification and quotation scope. Use the comparisons below to prepare a consistent brief and check what each quotation includes.
Compare extension quotations against the same design, specification, professional scope, VAT basis, allowances and exclusions. The tables below are quotation checklists, not fixed-price schedules or regional price benchmarks.
House Extension Cost Comparison by Type
Extension type changes the scope, but it does not create a reliable universal price ranking. Compare the actual structure, footprint, access, roof, services, planning route, specification and professional input for each option.
| Extension type | Design information | Scope to price | Items to compare |
|---|---|---|---|
| Single-storey rear extension | Confirm the footprint and drawings | Ask for groundworks, structure and roof costs | Check glazing, services and room fit-out inclusions |
| Double-storey rear extension | Confirm both floor layouts and structural design | Ask for foundations, floors and roof scope | Check upper-floor access and making good |
| Single-storey side extension | Confirm side access and boundary constraints | Ask for structural openings and services scope | Check drainage, external works and access costs |
| Side-return extension | Confirm the side-return layout and roof design | Ask for structural openings and glazing scope | Check drainage changes and making good |
| Wrap-around extension | Confirm the combined rear and side layout | Ask for the complete structural and roof scope | Check services, finishes and external works |
| Two-storey side extension | Confirm both floors and the planning route | Ask for foundations, structure and roof scope | Check stairs, services and professional fees |
* TCM prices are project-specific. Confirm VAT treatment, professional fees, finishes, allowances and exclusions in the actual quotation.
House Extension Cost Scope by Size
Record the floor area and agreed specification before comparing quotes. Use the checklist below to separate the base scope from optional changes and identify what each contractor has included.
| Quote item | Defined specification | Alternative to price | Inclusions to check |
|---|---|---|---|
| Structure and openings | Use the same drawings and dimensions | Ask for each design change to be priced separately | Groundworks, structural work and making good |
| Roof and glazing | List the roof build-up and glazing schedule | Compare named products and agreed sizes | Rooflights, doors, drainage and installation |
| Services and finishes | List fixtures, finishes and service points | Separate optional upgrades from the base scope | Labour, materials, testing and completion work |
* Confirm VAT treatment, professional fees, external works, allowances and exclusions for the individual project.
What Affects the Cost of a House Extension?
A per-m² figure can hide important differences in scope. Compare the actual design, structural work, access, services, specification, professional input, VAT basis, allowances and exclusions.
Floor area affects quantities, but unit cost is also shaped by structure, access, services and specification.
Glazing, joinery, kitchens, bathrooms, finishes and performance requirements can materially change scope and cost.
Openings, beams, foundations, temporary works and ground conditions require project-specific structural input where applicable.
Parking, deliveries, storage, waste, working restrictions and site access can affect preliminaries and programme.
Design, structural, planning, building-control and other professional or statutory costs depend on the actual appointments and route.
Party Wall etc. Act requirements depend on the actual work and relationship to adjoining structures; a distance alone does not determine applicability.
Cost Breakdown: Labour, Materials & Professional Fees
A useful quotation separates labour and trades, materials, professional and statutory costs, site setup and exclusions. Ask for allowances and optional work to be identified separately.
Define included trades and specialist responsibilities
Define products, quantities, allowances and substitutions
Define appointments and deliverables
Define application, inspection and evidence requirements
Define access, waste, storage, temporary works and programme assumptions
How to Get an Accurate Quote for Your Extension
Online cost guides can show which factors matter, but they cannot establish a TCM price for a specific project. The information needed for a reliable quotation depends on the property, design maturity, access, surveys and specialist inputs; a site visit is not presented here as universally required or free.
When requesting quotes, always ask for an itemised breakdown — not a single lump sum. This allows you to compare quotes meaningfully and understand exactly what is and is not included. Ask specifically whether the quote includes VAT, professional fees, Building Regulations, and external works.
Compare quotations on the same defined scope, assumptions, allowances, VAT treatment and exclusions. Check relevant evidence, insurance appropriate to the work, responsibilities and change-control terms rather than assuming one pricing model is universally preferable.
Contact TCM to discuss your proposed extension. Confirm availability for the property address, the information needed for assessment, any visit or design fees, and the inclusions and exclusions in the quotation.