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by Sean Hill on Mar 19, 2026
So you've decided to extend. Now comes the question nobody gives you a straight answer to.
Search online and you'll find ranges so wide they're almost meaningless - "£30,000 to £150,000" - which tells you nothing useful about your specific project, your street, or your ambitions. This post won't do that to you.
What follows is an honest breakdown of what a house extension in London actually costs in 2026, what drives those costs up or down, and how to think about the investment before you speak to anyone.
A rear extension to a Victorian terrace in London - a glazed pavilion with sculpted timber arches opening the house fully to the garden. Projects of this ambition and specification sit at the higher end of the cost ranges explored in this guide.
Start with the right number: cost per square metre
The most reliable way to think about construction cost is cost per square metre of new internal floor area (GIA - gross internal area).
Construction costs increased significantly through 2020 and 2021 due to a perfect storm of COVID-related supply chain disruption and the post-Brexit reduction in available skilled labour - two pressures that hit the construction industry simultaneously and hard. Costs have continued to rise since, and as of 2026 remain stubbornly high - the anticipated drop has not materialised. In London, a well-specified rear or side extension sits broadly in these ranges:
| Specification level | Cost per m² (construction only) |
|---|---|
| Standard | £2,750 - £3,050 |
| Good quality | £3,050 - £3,750 |
| High specification | £3,750 - £4,250+ |
A typical single-storey rear extension adding 25-35m² of space will therefore carry a construction cost of roughly £70,000 - £120,000 at good quality specification. If you're adding a basement, budget from around £6,000 per m² for that element alone - it's a fundamentally different type of construction.
These figures are for construction only. They do not include architect fees, planning, structural engineering, or any number of other costs you'll encounter along the way.
The full picture: every cost you need to account for
Architect fees
A full architectural service from RIBA Stages 1-6 (brief through to completion on site) is typically calculated as a percentage of construction cost, though many practices charge a fixed fee agreed at the outset. Either approach is valid - what matters is that the scope of service is clearly defined.
Be wary of unusually low fees in the early stages: they often reflect a service that ends at planning, leaving you to manage technical design and construction yourself.
Full service includes concept design, planning drawings and application management, detailed technical drawings, structural coordination, contractor tendering, contract administration, and site inspections through to practical completion. That's a substantial body of work, and it's the difference between a project that runs smoothly and one that doesn't.
Planning application fees
A householder planning application in England now costs £548 from 1 April 2026 - up from £528 introduced in April 2025. Planning fees are now indexed to inflation annually, so expect this figure to continue rising.
The fee itself is modest relative to total project cost. The time cost is not - a standard householder application takes 8 weeks from validated submission to decision, and longer in some London boroughs. Factor in pre-application advice (worth doing for anything complex), and a realistic timeline from instruction to planning decision is 4-6 months.
Structural engineer
Almost every extension requires a structural engineer - for steel beam calculations, foundation design, or both. Budget £1,500 - £4,000 depending on complexity. Bear in mind that if your site has constrained access, proximity to protected trees, or neighbour foundation issues, specialist foundation work (such as mini piles) will add cost at both the engineer and contractor stage.
Party wall surveyor
If your extension is within 3 metres of a shared boundary or involves work to a shared wall, the Party Wall etc. Act 1996 requires you to serve notice on your neighbours. If they appoint their own surveyor, you pay both sets of fees. Budget £1,000 - £3,000, though costs can escalate if disputes arise.
Building control
Building regulations approval is separate from planning permission - and both are required. Local authority building control fees for a typical extension run £900 - £1,500. This covers the structural, thermal, fire safety, and drainage requirements of the build.
Contractor preliminaries and contingency
A good contractor's quote will include preliminaries - site setup, scaffolding, waste removal, insurance, project management overhead. These typically add 12-18% on top of the base labour and materials cost.
On top of that, hold a 10-15% contingency on the total build cost. Not because something will necessarily go wrong, but because extensions involve opening up existing structures - and existing structures contain surprises. An unexpected drainage run, a wall that isn't where the drawings suggested, insulation that needs upgrading to meet current regulations - these are common, not exceptional.
Finishes and fittings
Construction cost figures do not include kitchen units, bathroom sanitaryware, floor finishes, lighting, joinery, or decoration. These are often the costs that most surprise first-time clients.
As a guide: a good kitchen can be achieved for around £18,000 using quality carcasses with well-specified external finishes. Brand-name kitchens can reach £50,000 or more, but that premium rarely translates into proportionate value. Define your specification early and hold a realistic allowance from the start.
A worked example: whole-house renovation, dormer extension, and EnerPHit deep retrofit - North West London
A single-storey kitchen extension is one end of the spectrum. At the other end is the type of project we're delivering more frequently: a whole-house renovation that combines a ground floor side and rear extension, a dormer loft conversion, and a deep energy retrofit to EnerPHit standard - the retrofit equivalent of Passivhaus certification.
This is a genuinely transformative scope of work. It takes a typical Victorian terrace - cold, inefficient, spatially compromised - and turns it into a high-performance home that's warmer, quieter, healthier to live in, and dramatically cheaper to run.
The figures below are drawn from a real RISE project on a four-bedroom Victorian terrace in North West London. The existing house was 123.5m² GIA. The proposed scheme - adding a side extension, rear extension, and dormer - brings the total to 170m², with the entire building retrofitted to EnerPHit standard throughout.
At that scope and specification, the all-in construction cost sits at around £5,400 per m² of proposed GIA - reflecting not just the new build area but the full complexity of working within and transforming an existing Victorian structure while achieving certified low-energy performance throughout.
Construction works
| Item | Approximate cost |
|---|---|
| Preliminaries (site setup, scaffolding, hoarding, project management) | £60,000 - £70,000 |
| Demolition and stripping out | £33,000 - £38,000 |
| Structural works (foundations, steels and timber frame across all floors) | £75,000 - £85,000 |
| Building work and carpentry (floors, walls, roofs, external envelope) | £135,000 - £150,000 |
| External doors and windows (high-performance, triple glazed) | £65,000 - £72,000 |
| Drainage (above and below ground, rainwater goods, waterproofing) | £16,000 - £20,000 |
| Electrical (full rewire, solar PV, EV charger, home battery) | £58,000 - £68,000 |
| Heating and plumbing (ASHP, UFH, wet pipework, water treatment) | £60,000 - £68,000 |
| Carcassing, joinery and kitchen | £150,000 - £165,000 |
| Plastering | £42,000 - £47,000 |
| Finishes (polished concrete, engineered timber, tiles throughout) | £44,000 - £52,000 |
| Decorating | £48,000 - £54,000 |
| Sanitaryware and brassware | £28,000 - £32,000 |
| External works (patios, fencing, landscaping) | £13,000 - £17,000 |
| Construction subtotal | £850,000 - £950,000 |
Low-energy systems - broken out separately
These costs sit within the construction subtotal above but are worth understanding individually - both because they transform how the building performs, and because several qualify for reduced VAT at 0% under current HMRC rules:
| System | Scope | Approximate cost |
|---|---|---|
| Air source heat pump (ASHP) | Supply, install, MCS certification, dedicated pipework and fuseboard | £20,000 - £23,000 |
| Mechanical ventilation with heat recovery (MVHR) | Supply, install, full duct routes, terminals, commissioning to AECB CarbonLite standard | £17,000 - £20,000 |
| Airtightness package | ProClima membranes, tapes and grommets throughout, two independent air permeability tests | £13,000 - £15,000 |
| Thermal and acoustic insulation | Insulated plasterboard to all external walls and roof plane | £9,000 - £11,000 |
| Solar PV, battery storage and EV charger | 6-panel system, inverter, home batteries, 22kW EV charger | £12,000 - £15,000 |
| Underfloor heating | Wet UFH to ground floor, electric UFH to bathrooms | £12,000 - £14,000 |
| Low-energy systems subtotal | £83,000 - £98,000 |
These systems deliver the EnerPHit outcome: airtightness below 1.0 ACH, heat recovery above 85%, and a building heated entirely by a low-carbon heat pump. Energy bills on completion typically fall by 70-80% compared with a pre-retrofit Victorian terrace.
Professional fees and statutory costs
| Item | Approximate cost |
|---|---|
| Architect fees | £70,000 - £120,000 |
| Structural engineer | £6,000 - £9,000 |
| Mechanical and electrical engineer (heating design specialist) | £3,500 - £5,000 |
| Energy consultant and PHPP modelling | £3,000 - £4,500 |
| Party wall surveyor | £2,500 - £4,000 |
| Planning application | £548 |
| Building control | £1,200 - £1,600 |
| Professional fees subtotal | £87,000 - £145,000 |
Fit-out and finishes
| Item | Approximate cost |
|---|---|
| Kitchen (cabinetry, worktops, appliances) | £30,000 - £60,000 |
| Sanitaryware and brassware | £26,000 - £32,000 |
| Bespoke bedroom joinery | £40,000 - £50,000 |
| Fit-out subtotal | £96,000 - £142,000 |
(Note: floor finishes and decorating are included in the construction subtotal above.)
Contingency
On a project of this complexity - existing Victorian structure, full gas removal, EnerPHit airtightness requirements - a contingency of 15-20% of construction cost is appropriate: roughly £130,000 - £190,000.
Total project cost: approximately £1.0m - £1.15m
That figure warrants context. This is not a cosmetic renovation. It is a complete structural, spatial, and energetic transformation - removing gas entirely, achieving certified airtightness, installing heat recovery ventilation, and generating the home's own electricity. On completion, the house performs to EnerPHit standard and will do so for decades. Energy bills fall dramatically. The embodied carbon in the existing structure is retained rather than demolished.
Against the cost of purchasing an equivalent home in the same area - plus stamp duty, legal fees, and moving costs - the numbers often tell a compelling story for staying and investing.
What drives costs up
- Basement work - from around £6,000/m² for the basement element alone
- Constrained or difficult sites - party wall complications, narrow access, parking permits
- Listed building status - specialist contractors, more exacting specification, longer consents
- Conservation area constraints - limiting material choices and affecting programme
- Poor existing condition - older houses reveal surprises once opened up
- Full gas removal - terminating the mains supply and converting to all-electric adds cost but is standard on EnerPHit projects
- Non-standard construction - cross-laminated timber has real environmental advantages but can add around 10% compared with standard blockwork
What keeps costs down
- Clear, complete drawings before going to tender - contractors price risk; reduce uncertainty and they price more keenly
- Realistic specification from the start - scope creep mid-build is the single most consistent driver of cost overrun
- Fabric-first design - getting insulation, airtightness, and structure right before spending on systems saves money over the building's lifetime
- Early QS involvement - a quantity surveyor reviewing cost implications as the design develops produces reliably better outcomes than one reviewing a completed set
A note on VAT
Extensions to existing homes are subject to standard rate VAT at 20% on construction costs - already included in the contractor quotes you will receive.
One important exception: certain energy-saving measures - insulation, air source heat pumps, solar panels, MVHR systems - currently attract a reduced VAT rate of 0% under HMRC rules. On a project with the low-energy systems scope described above, this saving is meaningful and worth confirming with your contractor at tender stage.
If your project involves the amalgamation of two flats into a single dwelling, the construction works may qualify for a reduced VAT rate of 5% rather than the standard 20% - a significant saving on a large project. This is a nuanced area of VAT legislation and the specifics of your project will determine whether it applies, so always take advice from your accountant or a VAT specialist before proceeding.
Is it worth it?
This is the question only you can answer - but good architecture helps you think it through clearly.
In London's property market, a well-designed extension that adds usable, light-filled space typically adds value in excess of its cost - provided the design is good and the workmanship is sound. A whole-house EnerPHit retrofit adds something more: a demonstrable, certifiable performance standard that will only become more valuable as energy costs rise and buyers become more discerning about building quality.
More than either of those things, it adds daily quality of life. Warmth without draughts. Silence. Clean air. Spaces that work for the way you actually live. That's not easy to price - but it's real.
Where to start
The most important first conversation is with an architect - not a contractor, not a planning consultant - because the design shapes everything that follows: what's possible, what it will cost, and whether it will be worth living in.
At RISE, our initial consultations are a genuine two-way conversation. We want to understand your project, your home, and your priorities - and give you an honest assessment of feasibility and cost before any commitment is made.
Building for the future
At RISE, we believe that extending or retrofitting your home isn't just a construction project. It's an opportunity to create something that lasts - a home that performs beautifully, treads lightly, and genuinely improves the way you live every day.
Whether you're planning a simple rear extension or a whole-house transformation to EnerPHit standard, every project we take on begins with the same question: how do we make this building as good as it can possibly be - for the people who live in it, and for the world outside it.
Thinking about extending or retrofitting your London home? Let's talk about what's possible.
→ Email us at architects@risedesignstudio.co.uk
→ Or call the studio on 020 3947 5886
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