How Much Does a House Extension Cost in London in 2026?

A practical builder’s guide to construction costs, VAT, professional fees and the items most often missed.

The short answer is that there is no reliable extension price based on floor area alone. For early budgeting in London, however, a sensible allowance for the main construction work is approximately £3,000–£5,000+ per square metre, excluding VAT and professional fees.

That means a 20–30 m² single-storey extension may require roughly £60,000–£150,000 for the main build before VAT. Once design and statutory fees, contingency, the kitchen or other owner-selected finishes, and VAT are added, the total cash budget can easily move into the region of £90,000–£200,000+ depending on the scope and specification.

Quick answer

Budget £3,000–£5,000+ per m² for the main construction work in London, excluding VAT and professional fees. Small, structurally complex or access-restricted extensions can cost more per square metre than larger, simpler projects.

Five points to understand before comparing prices

• A cost-per-square-metre figure is a planning tool, not a quotation.

• Every headline figure should state whether VAT, professional fees, kitchens and finishes are included.

• A small side return can cost more per square metre because foundations, steelwork, drainage and mobilisation are fixed costs.

• Planning drawings alone may not contain enough detail for contractors to price on a like-for-like basis.

• Some work to a qualifying home empty for at least two years may be eligible for 5% VAT rather than 20%—but the relief is conditional, not automatic.

London house extension costs per square metre

The table below converts the £3,000–£5,000+ planning range into simple examples. It is deliberately presented as a main-construction allowance so that VAT and other project costs are not hidden inside an undefined ‘all-in’ figure.

Specification / complexity	Indicative build rate	20 m²	25 m²	30 m²
Cost-conscious / straightforward	£3,000 per m²	£60,000	£75,000	£90,000
Mid-range	£4,000 per m²	£80,000	£100,000	£120,000
Higher-specification / complex	£5,000+ per m²	£100,000+	£125,000+	£150,000+

Basis: indicative London main-construction cost only, excluding VAT, professional/statutory fees, contingency, loose furniture, temporary accommodation and any kitchen, appliances or finishes identified as client-supplied or separately priced.

The lower rate is most achievable with a simple shape, good access, standard products and a settled design. The upper end becomes more likely with large structural openings, bespoke glazing, restricted access, extensive alterations to the existing house or a premium interior specification.

Build cost and total project budget are not the same

A homeowner can receive a perfectly valid builder’s quotation and still underestimate the amount of money needed to complete the project. A realistic cash budget normally considers the following layers.

Budget layer	What it may include	What to check
Main construction	Site setup, demolition, groundworks, structure, envelope, services, plastering and agreed finishes	Exact inclusions, exclusions and provisional sums
Professional and statutory fees	Architect, structural engineer, planning, building control, party wall and specialist surveys	Which consultants are appointed directly by the client
Kitchen and selected finishes	Cabinetry, worktops, appliances, flooring, sanitaryware, lighting and joinery	Whether supply, installation or both are included
VAT	Usually 20% on work to an existing dwelling; limited reliefs may apply	Whether every quoted figure is net or gross
Contingency	A reserve for conditions or necessary work that could not reasonably be confirmed before opening up	Keep separate from optional upgrades
Living and external costs	Temporary accommodation, storage, garden reinstatement, parking, permits and insurance	Costs outside the building contract

A 25 m² budgeting example

At £4,000 per m², the main build is £100,000 excluding VAT. Add £20,000 standard-rate VAT, an illustrative £15,000 gross allowance for design/engineering/approvals, and a £10,000 cash contingency, and the working budget becomes about £145,000 before a separately purchased kitchen, appliances, landscaping or temporary accommodation. This is an illustration—not a quotation—but it shows why ‘£4,000 per m²’ does not mean the whole project will cost £100,000.

Why extension type changes the price

Extension type	Typical cost pressures
Straightforward rear extension	Usually the clearest geometry, but glazing, structural opening size, drainage and garden access remain important
Side return	Small footprint, restricted working area, drainage diversions, roof glazing and substantial steelwork can create a high cost per m²
Wraparound	Larger footprint, more complex roof and structural junctions, and greater alteration to the existing rear/side walls
Double-storey	Shared foundations and roof can improve value per m², but planning, fire, access and structural requirements are more involved
High-specification kitchen extension	The shell is only part of the cost; cabinetry, worktops, appliances, lighting, flooring and services can add tens of thousands of pounds

What affects the cost of a London extension?

Structural complexity

Removing an external wall to form an open-plan kitchen or living area may require steel beams, columns, temporary support, padstones and strengthening to existing foundations. The cost depends on the engineer’s final design—not simply the span.

Access and logistics

Terraced houses with no side access may require materials and waste to pass through the home. Parking restrictions, skip permits, material storage, concrete delivery, scaffolding and restricted working hours all affect labour and preliminaries.

Existing property condition

Older London homes may contain damaged drains, shallow foundations, damp, ageing services, hidden pipework or previous unsupported alterations. Surveys reduce uncertainty, but some issues are only visible after demolition.

Glazing and external doors

Bespoke rooflights, slim-frame sliding doors and large glazed openings can transform the design, but their manufacture, structural support, lead time and installation can form a significant part of the budget.

Kitchen and interior specification

A kitchen extension is not just an empty room. Plumbing, electrics, cabinetry, appliances, worktops, floor finishes, lighting and heating choices must be coordinated before the construction budget is finalised.

Changes to the existing house

New and existing areas must work together. Making good adjoining floors and ceilings, moving doors, upgrading heating or electrics and redecorating circulation areas can extend the scope beyond the new footprint.

Can VAT be 5% if the property has been empty for two years?

Potentially, yes. Most work to an existing house or flat by a VAT-registered contractor is charged at 20%. HMRC allows a reduced rate of 5% on qualifying renovation or alteration work to eligible residential premises that have not been lived in during the two years immediately before that contractor’s work starts.

HMRC’s guidance states that qualifying improvement work carried out during the renovation of the empty property can include construction of an extension, together with qualifying building materials supplied and installed by the contractor. The relief is not automatic, and it does not make every project cost 5%. Professional services, landscaping, scaffolding, hire and goods that are not building materials can remain standard-rated. Materials bought directly by the homeowner will generally carry their normal VAT rate.

Raise the issue before the contract is agreed. The contractor must be satisfied that the conditions are met and should keep reliable evidence, which may include Council Tax or electoral records, utility information or confirmation from the local authority’s Empty Property Officer. The timing matters: occasional residential use during the relevant two-year period can prevent the relief from applying.

Why this is worth checking

On £80,000 of fully qualifying contractor work, 20% VAT is £16,000 while 5% VAT is £4,000—a potential difference of £12,000. Eligibility and any necessary apportionment should be confirmed against HMRC Notice 708; homeowners should not simply ask for a 5% invoice without supporting the conditions

Official guidance: HMRC Buildings and construction (VAT Notice 708), section 8. Tax treatment depends on the facts of the individual project, so specialist VAT advice may be appropriate.

Costs and approvals homeowners often overlook

Party Wall matters

The Party Wall etc. Act 1996 may apply to work on a party structure, building at or astride a boundary, or certain excavations near neighbouring buildings. Notice may be required before work begins, and the building owner will commonly meet reasonable surveyor costs where a dispute procedure is needed.

Drainage and build-over approval

Public sewers often run through London rear gardens. In the Thames Water region, an application is likely when building within 3 metres of a public sewer or within 1 metre of a public lateral drain. Surveys, design changes or diversion work can affect cost and programme.

Building control and technical design

Planning permission and Building Regulations are separate. Most extensions require Building Regulations approval, structural information and inspections, even when the extension is permitted development.

Leasehold or lender consent

A ground-floor flat or leasehold house may need freeholder consent or a licence to alter in addition to statutory approvals. Mortgage providers and insurers may also need to be notified.

Temporary living arrangements

Noise, dust and interruptions to the kitchen, heating, water or electricity may make temporary accommodation, storage or a temporary kitchen necessary.

External works and reinstatement

Foundations, drainage and access can disturb patios, planting, fences and lawns. Make clear whether landscaping and full reinstatement are included.

Utility upgrades

A larger kitchen or living area may require boiler or hot-water changes, consumer-unit work, additional heating, meter relocation or drainage alterations.

Do you need planning permission?

Not every house extension requires a full planning application. Some extensions to houses can be permitted development when all limits and conditions are met. Those householder rights do not apply to flats and maisonettes, and they may be limited or removed by a planning condition, an Article 4 direction, the property’s planning history or its location. Listed buildings and conservation areas need particular care, and listed building consent is a separate consideration.

Building Regulations approval is separate from planning and is required for most extensions. Party Wall procedures, freeholder consent and a water-company build-over agreement are also separate; obtaining planning permission does not replace them.

Check current guidance: Planning Portal – extensions; GOV.UK – Party walls and building work; and, where relevant, Thames Water – build over agreements.

How much contingency should you allow?

For many residential extensions, keeping 10–15% of the construction budget as a separate contingency is a sensible starting point. A higher allowance may be prudent where the design is unresolved, the property is old or poorly surveyed, access is difficult, or extensive opening-up is required.

Construction budget	10% contingency	15% contingency
£70,000	£7,000	£10,500
£100,000	£10,000	£15,000
£130,000	£13,000	£19,500

Contingency is not an allowance for upgrades. Keep optional choices—such as a more expensive kitchen or upgraded glazing—in a separate client-change budget

How to obtain a more accurate quotation

The quality of a quotation depends on the quality and consistency of the information issued to contractors. A planning drawing may show the appearance of the extension but still leave important construction and specification decisions unanswered.

For a meaningful like-for-like quotation, provide as much of the following as is available:

• planning and Building Regulations / construction drawings;

• structural engineer’s drawings and calculations;

• drainage information and any build-over requirements;

• window, external-door and rooflight schedules;

• kitchen layout and service requirements;

• finishes schedule, including who supplies each item;

• clear scope of work and exclusions;

• planning, Party Wall and freeholder status;

• site access, parking and working-hour restrictions;

• target programme and budget range.

What should you check in a contractor’s quotation?

• Is VAT included, excluded or applied differently to separate parts of the work?

• Are structural steelwork, temporary works, drainage and utility alterations included?

• Which kitchens, appliances, sanitaryware, flooring, decorating finishes and light fittings are client-supplied?

• Are provisional sums realistic, and how will any difference be dealt with?

• How will variations be instructed, priced and approved?

• What are the payment stages and expected construction programme?

• Who is responsible for building control, Party Wall matters, parking, skips and permits?

A lower quotation may simply omit work included elsewhere. Compare the scope and assumptions behind each total before comparing the totals themselves.

Frequently asked questions

How much does a 20 m² extension cost in London?

As an early 2026 allowance, the main construction work may be about £60,000–£100,000+ excluding VAT and professional fees. The total cash budget can be materially higher once design, approvals, contingency, kitchen/finishes and VAT are included.

How much does a 30 m² extension cost in London?

A broad main-build allowance is approximately £90,000–£150,000+ excluding VAT and professional fees. Complex steelwork, restricted access, bespoke glazing and premium finishes can take the project above that range.

Why can a side return be expensive per square metre?

Its footprint is small, but it can still need foundations, drainage alterations, roof glazing, extensive structural steelwork and work in a narrow access zone. Those fixed costs are spread across fewer new square metres.

Is the kitchen included in the extension price?

Not automatically. Some quotes include only the shell, services and installation connections; cabinetry, worktops, appliances and even installation may be separate. The quotation should state this explicitly.

How long does a house extension take?

A straightforward single-storey construction phase may take around three to six months. The full process can take six to twelve months or more once design, planning, technical approvals, Party Wall matters, procurement and fit-out are included.

Can I live in the house during the work?

Often yes, but expect dust, noise, reduced privacy and temporary interruptions to services. Whether it is practical depends on the sequence of work and whether the kitchen and principal access routes remain usable.

Is VAT normally charged on a house extension?

Yes. A VAT-registered contractor will normally charge 20% on extension work to an existing dwelling. A 5% reduced rate may apply to qualifying work on eligible residential premises that satisfy HMRC’s two-year empty-property conditions.

When should I contact a construction company?

An early discussion can test feasibility and budget, but a dependable fixed quotation usually requires coordinated drawings, structural information and a clear specification. You do not need to wait until every finish is chosen to make an initial enquiry.

Planning a house extension in London?

Alt Construction undertakes residential extensions and refurbishment projects across London. We work with the client’s appointed architect, structural engineer and other consultants to turn coordinated project information into a clear construction scope and quotation.

If you already have drawings, send the property postcode, current design information, intended specification, preferred timeframe and approximate budget range. If the project is still at feasibility stage, we can explain what information a contractor will need and, where appropriate, introduce independent design professionals.

Request an initial project discussion

Email info@altconstruction.co.uk or visit www.altconstruction.co.uk. A project-specific price can only be prepared after reviewing the property, drawings, specification, responsibilities and site conditions.

Important note

The prices in this guide are indicative planning allowances reviewed in July 2026. They are not a quotation, valuation or guarantee of final cost. Planning, Building Regulations and VAT treatment depend on the individual property and scope; obtain project-specific professional advice where required.