Free online calculator
House extension cost calculator for London
A single-storey house extension in London costs from £1,900 per m² including VAT: a simple 15 m² rear extension starts from about £28,500. Glazing, a kitchen fit-out, foundations and fees add to that, so enter your size and choices below to see your own budget.
Start the calculatorCall 07353 373753
- London rates, 2026 prices
- VAT included
- No sign-up to see the price
Calculator
Work out your house extension cost
Answer the questions below. The estimate updates as you go, and you only leave your details if you want an exact price.
House extension cost calculator
Enter the size and type of your extension, what it will be used for, glazing, lighting, kitchen and bathroom, and see a London price including VAT.
What is included
What the calculator counts
- The size of the extension.
- What the new space will be used for.
- Glazing and heating.
- The fit-out, including a kitchen or a bathroom in full detail.
- Piled foundations, demolition of an old conservatory, a porch, garden room or orangery, scaffolding.
- Planning fee, lawful development certificate, Thames Water build-over agreement and party wall awards.
- New partition walls, by length and height.
- Drawings and structural calculations can be uploaded with your request.
Price terms
What is in the price
Every estimate is for supply and fit: we buy the materials and fittings, do the work with our own team and clear up. This is what the price covers, what it leaves out, and what we need from you.
Included: what we supply and do
- Labour of our own plumbers, gas engineers, electricians, tilers, carpenters and decorators.
- Building materials: pipework, cables, consumer unit parts, plasterboard, plaster, adhesive, grout, screws, sealant, paint and primer.
- Foundations, walls, roof, insulation, bifold or sliding doors, rooflights, electrics, heating and the fit-out you choose.
- Every fitting you tick in the calculator at the finish level you choose (standard, mid-range or premium), bought by us and fitted.
- Waste removal and skips, protection of floors and furniture, site set-up and project management.
- Testing and certificates: gas safety, electrical (Part P) and building control notices where the work needs them.
- VAT at 20%: the range shown is the figure you would pay.
Not included
- Architect drawings, structural engineer, planning fees and party wall surveyors, unless you tick them in the calculator.
- Planning fees, build-over agreement and party wall awards unless ticked; garden reinstatement beyond the footprint.
- Structural problems found after opening up: rot, damp, asbestos, old wiring not up to standard. They are priced separately after the survey.
- Furniture, curtains, blinds, light bulbs in decorative fittings and loose appliances that are not built in.
- Parking permits and congestion charges, which are charged at cost.
From you
- If you prefer to buy your own tiles, sanitaryware, kitchen units or appliances, tell us: we fit them and take their cost out of the price. Supply-only items must be on site before the work starts.
- Access to the property, water and electricity during the work.
- Decisions on colours, tiles and fittings before the work starts, so nothing waits on a delivery.
Budget
What changes the price most
What the space is for
A kitchen extension costs more than a living room of the same size, because it adds units, plumbing, electrics and appliances to the building work.
Glazing
Large sliding or folding doors and roof lights cost more than standard windows and doors, and they change how much heating the room needs.
Fit-out level
The shell is only part of the budget. Flooring, plastering, decoration and fitted furniture are priced at the finish level you choose.
How it works
From estimate to fixed price
Answer the questions
Sizes, finishes and fittings. It takes a few minutes and you can change any answer.
See the estimate
A price range with VAT, with a breakdown of what it is made of.
Send it to us
Add your contact details, and a drawing if you have one, to ask for an exact price.
Free survey, fixed price
We visit, measure and confirm a fixed price before any work starts.
Read how we carry out home refurbishment in London, or see our hourly rates for smaller jobs on the pricing page.
Questions
House extension cost calculator questions
How much does a house extension cost in London?
A simple single-storey extension starts from £1,900 per m² including VAT, so about £28,500 for 15 m². The final cost depends on the size, what the space is used for, the glazing and the fit-out. The calculator prices each part and shows a range with VAT.
Do I need planning permission for an extension?
Some single-storey rear extensions fall under permitted development. Larger extensions, flats and properties in conservation areas need planning permission, and building regulations approval is always required. Check with your local council before work starts.
Can I upload my architect's drawings?
Yes. Upload a PDF or a photo, up to 10 MB, and it goes to our estimator together with your request.
Is the estimate a fixed quote?
No. It is a budget range. The fixed price is confirmed after a free survey.
What is included in the price?
Labour, every material and fitting you tick at the finish level you choose, waste removal, protection, project management, certificates and VAT. Drawings, planning fees, party wall awards and surveys are included only when you tick them in the calculator. The full list is in the section "What is in the price" on this page.
Can I buy the materials myself?
Yes. If you buy your own tiles, sanitaryware, units or appliances we fit them and take their cost out of the price. They must be on site before the work starts. Everything else, from pipework to paint, is supplied by us.
Why Plumbery
Qualified, insured and upfront about price
Every engineer we send is registered or qualified for the trade they work in, and you can check the details before anyone starts.
- Fully insuredEvery job is covered by our insurance.
- Rates published in advanceOur hourly rates are on the pricing page. Nothing starts until you agree the rate.
- Paperwork includedGas safety certificates and electrical records are provided on completion.
- Registered companyPlumbery Ltd, company no. 11677583, registered in England and Wales.
Renovation
Kitchens, bathrooms and extensions
One team for the fitting, plumbing, gas and electrics, with an itemised written quote before work starts.
