House extension cost in London

Possible additions (incl. VAT)

• New kitchenNxtFind estimate, indicative£12,500 – £39,000
• Bifold or sliding doorsNxtFind estimate, indicative£4,500 – £12,500
• Roof lantern or skylightsNxtFind estimate, indicative£3,000 – £9,500

On top of the build cost above, including VAT, London.

Example at a glance

• Estimated project cost£94,000 – £118,000 incl. VAT
• Build cost before VAT£78,000 – £98,000
• Not included: fees and permissions incl. VAT£7,000 – £27,500+
• Not included: contingency£9,000 – £18,000
• All-in budget incl. VAT, fees and contingency£110,000 – £163,500+
• Likely time on site12 – 18 weeks
• PlanningPermitted development or a full application. Worth checking early

Indicative only, not a quote. Based on published UK cost guides checked September 2026, adjusted for London. Includes VAT at 20%. Excludes professional fees, planning, building control, party wall costs, surveys and contingency.

Where the money goes (before VAT)

The indicative building work, £78,000 – £98,000 before VAT, split across the stages of the build, so you can see how each quote splits the work.

• Groundwork, foundations and drainage£16,500 – £20,500
• Structure, walls and steels£17,500 – £21,500
• Roof and waterproofing£9,500 – £12,000
• Windows, doors and glazing£9,500 – £12,000
• First fix: electrics, plumbing and heating£9,500 – £12,000
• Insulation, plastering and screed£9,500 – £12,000
• Decoration, flooring and making good£6,000 – £8,000

These stages split the building work only. A kitchen or other extra you selected is shown separately in the summary, and a kitchen you did not select is not included at all. Professional fees and landscaping are not included. NxtFind estimate, indicative: typical proportions for this kind of project. Yours will differ.

How long it takes

• Time on site12 – 18 weeksOnce work starts. Design, planning and finding the right builder usually take longer than the build itself.
• Planning and permissionsSingle-storey rearSome extensions fall under permitted development, subject to strict limits on depth, height, materials and how much of the garden is built on. Flats do not generally have the same householder permitted development rights, and conservation areas, listed buildings, Article 4 directions and previous extensions to the property can all affect what is allowed. Building regulations apply either way. Check with your council or a planning consultant before assuming which route applies.

Other costs to allow for (before VAT)

None of these are in the estimate above, and they sit outside a builder’s price. Budget for the ones that apply to you.

• Architect or designer£5,000 – £15,000Typically 7–15% of the build cost. Full service, from first drawings to site visits. Drawings-only packages cost far less.
• Structural engineer£500 – £2,000A single beam calculation sits at the bottom of that range. A whole-project package, or a site with awkward ground, costs more.
• Planning application£0 – £548The householder fee in England is £548, set nationally rather than by your council. Draft regulations propose £575 for applications made from 8 December 2026, subject to parliamentary approval. A lawful development certificate or a prior approval costs less, and work that genuinely falls under permitted development needs no application at all. A planning consultant, if you use one, is extra.
• Building control£600 – £1,500+Varies a great deal by council and by project, and again depending on whether you use the local authority or a registered building control approver. Larger jobs run higher still, and VAT may be added.
• Party wall surveyor, where applicable£800 – £3,600Where you share a wall or dig near a boundary. Around £800–£1,800 is typical where both sides agree to a single surveyor handling the matter. It can be materially higher, commonly £1,500–£3,600 per affected neighbour, where each side appoints its own surveyor, and it is charged per neighbour, so a terrace with neighbours on both sides costs more again.

Typical UK ranges, shown before VAT. London and the South East sit at the top of most of them. Professional fees and building control usually carry 20% VAT; planning fees don’t. Together they come to £6,000 – £23,000+ before VAT, or £7,000 – £27,500+ including VAT, the figure in the summary. Planning fees last checked 13 October 2026.