House Extension Cost in London (2026): Rear, Side Return and Wrap-Around

Last updated: September 2026. Planning, party wall, VAT and consumer-rights guidance verified against gov.uk, the Planning Portal, HSE, RICS, Citizens Advice and named London borough sources as of September 2026. Build costs are dated and sourced below.

TL;DR: A house extension in London costs roughly £2,300 to £4,500 per square metre to build in 2026, excluding VAT and professional fees. A typical 20 m² single-storey rear extension lands near £52,000 to £90,000. Design, engineering, party wall and statutory fees add roughly 15 to 20 per cent on top.

Extension quotes are the hardest numbers in home improvement to compare, because almost nobody quotes the same thing. One builder gives you a per square metre rate for a shell. Another gives you a fixed price that quietly excludes the kitchen, the glazing and the steel. A third gives you a figure plus VAT, plus fees, plus provisional sums. The headline numbers can look 40 per cent apart when the actual scope is nearly identical.

This guide separates the four layers you actually pay for: the build cost, the professional fees, the statutory costs, and VAT. It then sets out exactly what a fair quote itemises, so you can put three builders on the same page and compare like with like.

What a fair extension quote itemises

Read this before you read the prices. Most extension overruns are not caused by a builder being dishonest. They are caused by a quote that was never complete in the first place, and by the homeowner not noticing until the invoices arrive.

A quote you can actually rely on names all of the following, with a price or an explicit exclusion against each:

• Preliminaries: site set-up, welfare, skips, protection of the existing house, project management, and insurance.
• Scaffolding: how long it is on hire for, and who pays if the programme slips.
• Groundworks and foundations: excavation depth, concrete volume, disposal of spoil, and what happens if the ground is worse than assumed.
• Drainage: new runs, connections, and any manhole that must be moved or built over.
• Structure: steel beams by size, padstones, and the structural engineer's drawing they are priced from.
• Envelope and roof: wall build-up, insulation, roof construction, and the weathering details at the junction with the existing house.
• Glazing schedule: doors and rooflights by manufacturer, model, size and colour. Not "sliding doors, allowance".
• First and second fix: electrics, plumbing, heating, and whether the existing systems can take the extra load.
• Plaster and decoration: whether you are getting a plastered and painted room or a bare shell.
• Provisional sums: every item that is an allowance rather than a price, clearly flagged. These are the numbers that move.
• Exclusions: kitchen units and appliances, flooring, landscaping, and professional fees are commonly excluded. Get the list in writing.
• VAT: whether the figure shown includes it. The Federation of Master Builders specifically advises homeowners to confirm this, because quotes are presented both ways [18].
• Stage payments: tied to completed and inspected work, never to calendar dates, with a retention held back until snagging is done.

If a builder will not itemise to this level, that alone is useful information. Our guide on how to avoid cowboy builders covers the other warning signs worth knowing before you sign anything.

How much does a house extension cost in London?

The Federation of Master Builders puts the UK average for a fully finished single-storey extension at £1,800 to £3,000 per square metre, and states that London and the South East can cost 30 to 50 per cent more per square metre than the national average [1]. The HomeOwners Alliance puts London and the South East at £2,300 to £3,400 per square metre for a single-storey build [2].

Taken together, that gives a defensible London working range of roughly £2,300 to £4,500 per square metre for the build, before VAT and professional fees, from standard through to good specification. Architect-led schemes in inner London, with bespoke glazing and high-end finishes, do run above that band, but treat figures above £4,500 per square metre as a specification choice rather than a London baseline.

The FMB's most useful single data point is the common case: a 20 m² single-storey extension costs £40,000 to £60,000 plus VAT and professional fees [1]. Apply the London premium and the same job in London sits at roughly £52,000 to £90,000 on the same exclusions.

Cost by extension type

Figures are build cost only, excluding VAT, professional fees, statutory fees and kitchen fit-out. Per square metre rates are derived by applying the FMB London and South East premium [1] to the FMB and HomeOwners Alliance national bands [1] [2]; they are our calculation from those sources, not a direct quotation from either body.

Extension type	Typical size	Indicative London build cost	What drives the price
Single-storey rear, standard specification	15 to 20 m²	About £2,300 to £3,400 per m², so roughly £38,000 to £65,000	Simple rectangular footprint, standard doors, existing drainage, builder's finish
Side return on a Victorian terrace	12 to 18 m²	About £3,000 to £4,500 per m², so roughly £40,000 to £75,000	Structural steel to open up the existing wall, roof glazing, party wall on one or both boundaries, restricted access
Wrap-around or high-specification rear	25 to 35 m²	About £3,200 to £4,500 per m², so roughly £85,000 to £150,000	Large sliding doors, roof lantern, complex roof junctions, underfloor heating, higher finishes
Two-storey	30 to 50 m² over two floors	About £2,300 to £3,600 per m², so roughly £75,000 to £170,000	Cheaper per m² because foundations and roof are shared across two floors [2], but more total area and often a staircase alteration

Two honest caveats. First, no industry body publishes a London-specific rate for side returns or two-storey work separately, so those rows apply the same premium logic and should be read as indicative. Second, the kitchen is almost always outside the build cost: budget a further £15,000 to £35,000 for a mid to high specification kitchen installation including plumbing, gas and electrics [2]. Our kitchen fitting cost guide breaks that down separately.

If you are weighing going outwards against going upwards, our loft conversion cost guide prices the alternative on the same basis.

The costs that sit outside the builder's quote

This is the layer homeowners systematically underestimate. The FMB states that professional fees and compliance costs typically add a further 15 to 20 per cent on top of the base build cost [1]. On a £70,000 London build, that is £10,500 to £14,000 before a single brick moves.

Professional fees

Architect or architectural designer. RIBA explains that architects price by one of three methods: a percentage of construction cost, a lump sum, or a time charge [17]. RIBA also sets out how a fee is typically split across the work stages: 35 per cent for feasibility, concept design and planning, a further 35 per cent for technical design, and the remaining 30 per cent for construction and handover [17]. RIBA does not publish a percentage band, so treat the commonly quoted 8 to 15 per cent of build cost as market expectation rather than an official figure. Many London homeowners commission design up to planning only, then hand the technical package to a builder.

Structural engineer. Essentially every side return and most rear extensions need one, because opening up an existing load-bearing wall requires a specified steel beam. We could not find a figure from an approved industry body, so we are not going to invent one. Ask for a fixed fee quote covering calculations, drawings and a site visit, and expect it to be a four figure sum on a typical London job.

Party wall surveyor. Covered in its own section below, because on a London terrace it is close to unavoidable.

CDM duties. Under the Construction (Design and Management) Regulations 2015, a homeowner having work done on their own home has their client duties passed automatically to the contractor, or to the principal contractor where more than one contractor is involved, unless they agree in writing for a principal designer to take them on instead [12]. On a normal single-builder extension this needs no action from you. If you are directly contracting groundworkers, a steel erector and a fit-out team yourself, it does.

Statutory costs

Item	Cost	Notes
Householder planning application (England)	£548	Rose from £528 on 1 April 2026 [4]
Larger Home Extension prior approval	Set locally by each council	42 day determination, minimum 21 day neighbour objection window [19]
Building control, single-storey extension	About £650 to £1,550	Kingston charges £665 up to 10 m², £865 for 10 to 40 m², £1,095 for 40 to 100 m² [10]. Havering charges £859 up to 30 m², £1,183 for 30 to 60 m², £1,406 for 60 to 100 m² [11]
Skip permit	From about £65	Bexley charges £65 for up to 14 days, with a new application for each further period [15]
VAT	20 per cent	Standard rated on extensions to an existing home [9]

One correction worth making, because the wrong term circulates widely: a pavement licence is for hospitality furniture such as tables and chairs, and explicitly does not cover scaffolding, hoarding or skips [16]. Construction street works need a skip permit or a licence under the Highways Act 1980 from the borough instead. Ask your builder which they are obtaining and who is paying for it.

Planning permission and permitted development

Many London extensions do not need planning permission at all. The Planning Portal sets out the permitted development limits for single-storey rear extensions [3]:

• Depth: up to 3 metres beyond the original rear wall for a terraced or semi-detached house, up to 4 metres for a detached house.
• Larger Home Extension route: up to 6 metres for terraced and semi-detached, up to 8 metres for detached, subject to prior approval and neighbour consultation.
• Height: maximum 4 metres overall.
• Eaves: maximum 3 metres where the extension is within 2 metres of a boundary.

The Larger Home Extension route is not a planning application in the usual sense. The council notifies your adjoining neighbours, who get a minimum of 21 days to object, and the council has 42 days in total to determine it [19]. The fee is set by each council individually rather than nationally, so check your own borough's figure.

Where a full householder application is needed, the fee in England is £548 as of 1 April 2026 [4], and the council has a statutory 8 weeks to determine it, running from the date the application is validated rather than the date you submitted it [5].

Conservation areas and Article 4 directions

London has a great deal of designated land, and designation changes the rules materially. On designated land, which includes conservation areas, a rear extension of more than one storey always requires full planning permission, and cladding the exterior in stone, artificial stone, pebbledash, render, timber, plastic or tiles is not permitted development [3].

Separately, a council can make an Article 4 direction removing specific permitted development rights across all or part of its area. Many London boroughs have them, and they vary street by street. Check your own borough's Article 4 map and conservation area list directly on the council website before you assume anything is permitted development. This is the single most common source of nasty surprises on a London extension.

One further point for larger schemes: residential extensions creating less than 100 m² of new internal floorspace are generally exempt from the Community Infrastructure Levy, but charging schedules are set locally. A large two-storey extension approaching that threshold is worth checking against your borough's own CIL schedule.

Party wall notices on a London terrace

If you are building a side return, or excavating foundations near a neighbour's wall, the Party Wall etc. Act 1996 almost certainly applies. Notice is required when you work on an existing shared wall, build a new wall at or astride the boundary, or excavate within 3 to 6 metres of a neighbouring building's foundations depending on depth [6].

The timings matter to your programme:

• Two months' notice minimum for work to an existing party wall [6].
• One month's notice minimum for a new boundary wall or for nearby excavation [6].
• 14 days for your neighbour to consent in writing. Silence counts as dissent, not agreement [6].
• A served notice stays valid for one year [6].

If your neighbour dissents, each owner appoints a surveyor, or both agree a single surveyor, and the surveyors produce a binding Award setting out what work will be done, when, how, and who pays for what, usually alongside a schedule of condition recording the state of the neighbour's property beforehand [7].

The cost falls on you. RICS confirms that the building owner, meaning the person doing the work, normally pays the reasonable fees of all surveyors involved, including your neighbour's surveyor if they appoint their own [7]. The HomeOwners Alliance puts party wall surveyor costs at £900 to £2,700 for most standard projects, with hourly rates of £90 to £450 depending on location and complexity [8]. A single agreed surveyor acting for both parties is the cheaper route; two separate surveyors roughly doubles the bill, and you pay both.

On a terraced side return with neighbours on both sides, budget for two adjoining owners, not one.

VAT on a house extension

Building an extension to an existing home that remains a single dwelling afterwards is standard rated at 20 per cent VAT [9]. That applies to rear, side return, wrap-around and two-storey work alike, and it applies to both labour and materials.

The reduced 5 per cent rate exists, but it applies to converting a building into a house or flats, changing the number of dwellings, or renovating a property that has been empty for a qualifying period [9]. A normal extension to a home you already live in does not qualify. If you believe your project might, take advice against HMRC's VAT Notice 708 rather than relying on a builder's opinion.

Practically, the thing that catches people out is not the rate. It is that a quote of "£70,000" from a VAT-registered builder may mean £84,000. Ask, in writing, every time.

What moves the price up or down

Specification, not size, is the main reason the per square metre range is so wide [1] [2]. The specific drivers on a London job:

• Structural steel: every load-bearing wall you remove needs a beam, sized by an engineer. Wider openings and more of them mean more steel and more crane or manual handling on a tight site.
• Glazing: a full-width sliding door and a roof lantern can move a project from the middle of the range to the top of it on their own. This is the single easiest place to overspend or save.
• Drainage: where a public sewer runs under or within 3 metres of the footprint, you will need a build-over agreement from the water company, and possibly a diversion. Budget time as well as money.
• Ground conditions: London clay and made ground can require deeper foundations, and mature trees nearby, especially any with a Tree Preservation Order, can force deeper or piled foundations plus root protection.
• Access: many London terraces have no side access, so every load comes through the house. That is slower, and slower is more expensive.
• Specification creep: changing your mind after work starts is the most reliable way to turn a fixed price into a variable one. Fix the glazing and the kitchen before you sign.

Related trade costs are worth sanity-checking separately. See our London trade costs index for current rates, our plastering cost guide for finishing the new walls, and our house rewire cost guide if the existing consumer unit will not take the extra circuits.

How long an extension takes

The statutory elements are the ones you can plan around with confidence. A standard householder planning application has an 8 week determination period from validation [5]. The Larger Home Extension route runs to 42 days [19]. A party structure notice needs two months before work can start [6], and those two months can run in parallel with your planning period if you serve early.

On-site build durations are not published by any industry body we would cite, so we are not going to present a precise figure as fact. The realistic planning assumption for a London single-storey or side return extension is a build measured in months rather than weeks, with design, approvals and notices adding a similar period before it. Ask each builder for a programme with stage dates, and treat any promise of a very short build on a constrained terrace with scepticism.

How to get and compare extension quotes

1. Fix the specification first. Get a drawing package and a written scope before you ask for prices. Three quotes against three different assumptions tell you nothing.
2. Ask for three written quotes itemised to the checklist above, all against the same documents.
3. Compare the exclusions, not the totals. The cheapest headline figure is usually the one with the longest exclusions list.
4. Interrogate every provisional sum. Ask what the allowance covers and what the realistic cost is.
5. Check the contract and your rights. If you arrange the work in your own home rather than at the trader's premises, you have a 14 day cooling-off period in which you can cancel for a full refund. It starts the day after you authorise work to begin, and if a trader starts work during that period without your explicit approval, you keep the right to a full refund even after work has started [13].
6. Ask about financial protection. TrustMark registered businesses are required to provide financial protection, including an insurance backed guarantee for builds, which covers deposit protection and an insured workmanship warranty so you can still claim if the business stops trading [14].
7. Tie payments to completed work, never to dates, and hold a retention until snagging is signed off.

Frequently asked questions

How much does a single storey extension cost in London per square metre?

Roughly £2,300 to £4,500 per square metre for the build in 2026, excluding VAT and professional fees. That range applies the Federation of Master Builders' London premium of 30 to 50 per cent to the UK average of £1,800 to £3,000 per square metre [1], and matches the HomeOwners Alliance regional figure [2].

Do I need planning permission for a rear extension in London?

Often not. A single-storey rear extension up to 3 metres deep on a terraced or semi-detached house, or 4 metres on a detached house, with a maximum height of 4 metres, is usually permitted development [3]. Conservation areas and Article 4 directions remove those rights, so check your borough's own maps before assuming anything.

How much does a party wall agreement cost for a side return?

The HomeOwners Alliance puts party wall surveyor costs at £900 to £2,700 for most standard projects, with hourly rates of £90 to £450 [8]. As the building owner you normally pay all surveyors' reasonable fees, including your neighbour's if they appoint their own [7]. A terraced side return often involves two adjoining owners, so budget accordingly.

Is VAT charged on a house extension?

Yes. Extending an existing home that remains a single dwelling afterwards is standard rated at 20 per cent VAT on both labour and materials [9]. The reduced 5 per cent rate applies only to conversions, changes in the number of dwellings, or renovation of properties empty for a qualifying period, not to a normal extension of an occupied home.

What is the planning application fee for an extension in 2026?

The householder planning application fee in England is £548, having risen from £528 on 1 April 2026 [4]. The council has a statutory 8 weeks to determine the application, counted from the date it is validated rather than the date you submit it [5]. The Larger Home Extension prior approval route has a separate, locally set fee.

Who is responsible for site safety on my extension?

Under CDM 2015, a homeowner's client duties pass automatically to the contractor on a single-contractor job, or to the principal contractor where several contractors are involved, unless you agree in writing for a principal designer to take them on instead [12]. On a normal builder-led extension this requires no action from you.

Find a builder for your extension

An extension is the largest single job most London homeowners ever commission, and the quality of the builder matters more than any number in this guide. Compare three properly itemised quotes against the same drawings, check registrations and references, and put everything in writing before any money changes hands.

Prices in this guide are indicative and dated September 2026. Costs vary by borough, by property, by ground conditions and by specification, so treat every figure here as a starting point for a conversation rather than a quote. Find a trusted local tradesperson on Loacally, where every member is reviewed by hand before their profile goes live.

Sources

1. Federation of Master Builders, "Single-storey extension costs: a practical guide for homeowners", published 4 March 2026. https://www.fmb.org.uk/resource/single-storey-extension-costs-a-practical-guide-for-homeowners.html
2. HomeOwners Alliance, "House Extension Costs: Breakdown By Size & Type". https://hoa.org.uk/advice/guides-for-homeowners/for-owners/house-extension-costs/
3. Planning Portal, "Planning Permission: Extensions". https://www.planningportal.co.uk/permission/common-projects/extensions/planning-permission/
4. Planning Portal, "Application Fees England, 1 April 2026". https://ecab.planningportal.co.uk/uploads/english_application_fees.pdf
5. GOV.UK, "Determining a planning application", last updated 8 April 2026. https://www.gov.uk/guidance/determining-a-planning-application
6. GOV.UK, "The Party Wall etc. Act 1996: explanatory booklet", published 12 May 2016. https://www.gov.uk/government/publications/preventing-and-resolving-disputes-in-relation-to-party-walls/the-party-wall-etc-act-1996-explanatory-booklet
7. RICS, "Party walls" consumer guide, dated 24 February 2022. https://www.rics.org/consumer-guides/party-walls
8. HomeOwners Alliance, "Party Wall Surveyor Costs". https://hoa.org.uk/advice/guides-for-homeowners/i-am-improving/party-wall-surveyor-cost/
9. GOV.UK, "VAT for builders". https://www.gov.uk/vat-builders
10. Royal Borough of Kingston upon Thames, "Standard building control charges: domestic and residential". https://www.kingston.gov.uk/planning-and-building-control/building-control/submit-building-control-application/building-control-charges/standard-building-control-charges-domestic-and-residential
11. London Borough of Havering, "Building control charges". https://www.havering.gov.uk/info/20035/building_control/175/building_control_charges/3
12. Health and Safety Executive, "Domestic clients: roles and responsibilities", last updated 7 October 2025. https://www.hse.gov.uk/construction/cdm/2015/domestic-clients.htm
13. Citizens Advice, "Cancelling building or decorating work". https://www.citizensadvice.org.uk/consumer/getting-home-improvements-done/cancelling-building-or-decorating-work/
14. TrustMark, "Providing Financial Protection for Homeowners". https://www.trustmark.org.uk/ourservices/financial-protection
15. London Borough of Bexley, "Apply for a skip permit". https://www.bexley.gov.uk/bexley-business-employment/business-services/licensing-and-permits/building-and-construction-licences/skip-permits
16. GOV.UK, "Pavement licences: guidance", published 2 April 2024. https://www.gov.uk/government/publications/pavement-licences-guidance/pavement-licences-guidance
17. RIBA, "How do architects calculate their fees?". https://www.riba.org/explore/find-an-architect/homeowners-digest/how-do-architects-calculate-fees/
18. Federation of Master Builders, "Kitchen extension cost 2026: How much should you budget?", published 10 September 2025, updated February 2026. https://www.fmb.org.uk/resource/kitchen-extension-cost-2026-how-much-should-you-budget.html
19. North Hertfordshire Council, "Neighbour consultation scheme on house extensions", last updated 19 April 2024. https://www.north-herts.gov.uk/home/planning/planning-advice-and-guidance/neighbour-consultation-scheme-house-extensions