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How Much Does An Extension Cost

How much does an extension cost?


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Last updated Wednesday 27th October 2021

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Those looking to improve where they live have many options, but none have proven to be as popular as extending. With the increased desire for spacious, adaptable and functional homes, more are turning to extensions for the new lease of life they crave. But how much do they cost?

As with every home project, the overall cost of an extension will depend largely on what you are trying to achieve. Size, location and extension type are just a few of the many factors that will significantly affect the final cost. The price of a one-storey extension in Scunthorpe is going to be very different to the cost of a two-storey extension in London.

We’ve put together an explanation of the various factors that can affect the cost of an extension as well as give you an idea of how much you can expect your total budget to be.

How much does it cost to build an extension?

Ground floor rear extension (30 sqm)

Rear extensions are very popular extension types that build out of the back of the property into garden space.

Inside of London: £45,000 - £98,000
Outside of London: £37,000 - £80,000

Ground floor side return extension (30 sqm)

A side return extension (sometimes called a ‘side infill’) fills in dead alleyway space at the side of a property. These alleyways are commonly found attached to Victorian or period terraces, and are a good alternative to a rear extension as they do not sacrifice any garden space.

Inside of London: £52,000 - £98,000
Outside of London: £43,000 - £80,000

Ground floor wraparound extension (45 sqm)

If you’re really craving space you might decide to go for a wraparound extension. This combines a side return and rear extension in one big L shaped build that provides loads of space, but does take up some of the garden.

Inside of London: £78,000 - £146,000
Outside of London: £65,000 - £120,000

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Two storey

Two Storey





Two-storey extension (45 sqm)

Another way you can get more space if you’re not satisfied with a single extension is to extend on two storeys. These will cost just a little more than a single extension per square meter, but will obviously be much bigger in size, increasing the overall cost significantly.

Inside of London: £78,000 - £146,000
Outside of London: £65,000 - £120,000

Whole-house basement extension (80 sqm)

Basement extensions are a smart extension choice for properties in areas with high land values, such as London and other areas of the south. They are, however, more expensive due to the complexities of digging and building down.

Inside of London: £240,000 - £416,000
Outside of London: £198,000 - £343,000

Additional costs

It is important to note that the above estimates do not include VAT at 20%. They also do not include key fit-out items, such as kitchens, glazing, landscaping and bathrooms. The cost of these fit-out items will depend on the quality of finish you look to achieve. Typically they will take up around 5% of your overall budget.

The above estimates assume the existing property, soil, drainage and foundations are in a good/adequate condition. Encountering any issues in these areas will further affect your costs.

It is also assumed that existing rooms affected by the extension require decoration only. Refurbishing any existing rooms alongside your extension has not been accounted for in the estimated costs, and therefore the total SQM may not include everything that will need to be completed. The easiest way to calculate the additional cost of refurbishing existing rooms is to get a tailored estimate, as the prices will be very specific to your project.

Talk through your project in our free one-to-one consultations here.

Rear extension - Kitchen

Designed by Team Resi

What affects the overall cost?

As mentioned, the cost of an extension is affected by a number of factors. Here’s what you’ll need to consider with your extension project.

  • Size
  • Location
  • Function
  • Contractor
  • Materials
  • Design complexity
  • Soil, drainage and foundation condition


When delving into the costing of a project, most of your attention should be on construction. Construction and material costs will make up a great deal of your overall budget, and themselves are influenced by a number of variables.

One of the biggest variables comes down to who you get in to handle your bricks and mortar. Like with many industries, builders can be part of companies big and small, some are even operated by just a single contractor.

Here are the three types of construction companies you’re likely to encounter…

Individual - a sole trader working with subcontractors. Individual contractors are generally cheaper as they have lower overheads (no office, vans, admin team) but this can result in poorer communication and lack of proper paperwork.

Small company - typically 5 to 10 full time employees. These teams are established locally and may utilise contracts with admin support. This will typically result in the project running smoothly with a clearer level of communication and a conclusive paper trail. The costs for this type of contractor will reflect the support that they offer through utilising their admin support, vans and employees on payroll.

Large company - typically 10 to 20+ full time employees. Larger companies will have a strong online presence, several projects on site at any one time and loads of reviews you can look at. They are fully up to speed with contracts and will document all of the relevant paperwork effectively. They often provide a dedicated project manager, as well as having all trades on payroll from plumbers to roofers. All in all this added experience, manpower and professionalism will come at a premium.

Try our construction cost calculator tool and get a tailored construction estimate in minutes.

A wraparound extension - fully glazed

Designed by Team Resi

Where else will your money go?

So materials, fittings and labour costs will take up the bulk of your extension project budget. But what else? Here is a guide for working out where your full budget will go.

Architects - 4% (based on Resi’s services)
Administration fees - 1%
Structural Engineers - 3%
Surveyors - 2%
Contractors - 30%
Materials - 20%
Fittings - 5%
Glazing - 15%
VAT – 20%

Architectural Fees

For a traditional architect, fees can be between 5% - 15% of construction costs, depending on whether or not you bring them in to project manage.

At Resi, we tailor our prices to your project and can guide you from step one, all the way to getting ready to build.

Our packages include…

Other Professionals

Here are some of the other professionals you'll encounter on your home project journey.

Structural Engineer - one of the professionals you’ll meet during the building regulations stage is a structural engineer, who will make sure your project is strong, durable and safe. These can cost anywhere between £950-£2500.

CCTV Drainage Surveyor - as part of the building regulations stage you will also need a CCTV drainage surveyor to map drainage and sewer systems in the area you are building. These typically cost £250-£350.

Party Wall Surveyor - if you are building within 3 meters of a neighbour’s shared boundary wall, it is your legal duty to serve a party wall notice to the affected neighbours. If this is ignored or rejected, you will need to hire at least one party wall surveyor to resolve the issue. These typically charge £100-£140 an hour, and how long they work depends on the severity of the party wall dispute.

Approved inspector - these are alternatives to your local building authority. They work to the same standards, but can prioritise your project meaning the sometimes lengthy building regulations process is more streamlined. These typically cost between £900-£2000.

Side Return - Alex's Extension

Designed by Team Resi

Financing your extension with Resi

You’ve seen the numbers, now you might be wondering: how do I finance my dream extension?

This is where Resi Finance steps in. Thanks to our in-house experts, we provide…

  • Access to over 90 lenders including Lloyds, Barclays, NatWest, and Santander, so we can find the right deal for you.
  • We can arrange lending of up to 95% of the overall project costs, allowing you to take advantage of the future value of your property.
  • We’ll manage your application end-to-end to save you time and effort.
  • Provisional agreements in principle are available without affecting your credit rating.
  • High LTV and poor credit options are available.

See how much you can borrow with our Resi Finance calculator.


When it comes to extending your property, no matter what project you decide to go for there’s going to be a lot of variables which will affect the price of your build.

These include…

  • The size of your project
  • Complexity of the design
  • Your location
  • Materials you choose
  • Size of your chosen construction company
  • Plus the condition of your existing project and land

Therefore, if you’re serious about your extension, you should always take the time to get a tailored quote from an industry expert. Luckily, a free consultation with Resi is only a click away.

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