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How much does a side return extension cost?

Have you got a dead alleyway attached to your property? Think about building a side return extension and building out into that space. Here's how much you can expect to spend.

7 min read

If you have alleyway space to the side of your property and are looking to create some extra room, a side return extension might just be what you’re looking for. Side returns (sometimes referred to as side infills) allow you to expand out into an alleyway, turning an unused area into much desired living space.

The perks of a side return extension compared to other common extensions, like a rear extension, is that it tends to use up ‘dead space’, rather than the best parts of your garden. Ideal for anyone living in areas where green space is at a premium.

Types of side return extensions and how much they cost

There’s no one-size-fits-all when it comes to glass side extension costs because each property and project is so individual. While we can estimate amounts, it’s not an exact science and will depend on a number of factors – from the size of the extension to the specific materials used, contractors and area you live in.

Single-storey side return extension (3m x 5m wide - approx £90-120K)

A single-storey side extension is when unused space to the side of your house is filled in to create more room on one ground level of your property.

Double-storey side return extension (2m wide x 7m - approx £150-200K)

Much like a single-storey side extension, a double-storey repurposes unused space at the side of a house to create more room but over two floors, rather than one so your space is really maximised.

Side return kitchen extension (approx £80-120K)

A side return kitchen extension follows the same method as both single and double-storey extensions, but is specifically focused on creating more kitchen space. This allows for the possibility of reimagining the entire kitchen layout to make it multi-functional and more open-plan.

Side return extension for Victorian terrace (approx £80-120K)

Side returns are often applied to Victorian terraced properties as a lot of them possess alleyways in their configuration. By filling in this side alleyway, L shaped properties are squared off, creating that all important additional space.

Glass side return extension (approx £80-140K)This is when your side return extension is built with large glass panes to optimise the amount of light that reaches into your property. There are a number of different side extensions that include glass so, before you embark on your project take a look through projects on Pinterest to get a feel for what style you like.

Where will your money go?

Contrary to popular belief, there’s more to extensions than just materials and builders. This breakdown of costs should give you a rough idea of all the different areas your budget will need cover...

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%


The construction costs of a side return extension depend on a number of factors, including:

  • Build size
  • Design complexity
  • Property location
  • Professionals used
  • Building materials

For this reason, we suggest seeking out a construction cost estimation tailored to your project. For a tailored construction estimate in just a few minutes, try our construction cost calculator. To talk about your project with one of our professionals, you can also book a free consultation here.

If you’re only in the early stages of exploring your options, however, you might benefit from looking at some more general cost estimations. In the table below, you can see the construction estimates for a side return extension (excluding VAT) based on the type of contractor you hire.

Why have we done this? Individual contractors are cheaper, but can be slower without the access to additional resources and strong business relationships that you would likely get with a larger construction company. However, additional knowledge, manpower and experience will come at a premium.

Contractor London Estimate (Ex. VAT) Outside London Estimate (Ex. VAT)
Low end High end Low end High end
Individual £28,000 £40,000 £23,100 £33,000
Small Company £33,600 £48,000 £27,700 £39,600
Large Company £36,400 £52,000 £30,000 £43,000

(For a 16 sqm side return extension)

Note: these cost estimates are excluding VAT, which is currently 20%. They also do not include key fit-out items, such as kitchens, glazing, landscaping and bathrooms, and they assume the existing property, soil, drainage and foundations are in a good/adequate condition. Issues in these areas will further affect your costs.

During an extension, you might also decide to refurbish existing rooms outside of the project, making the most out of the contractors while they are in. While we think this is a great idea, it is important to note that the cost of refurbishing is not reflected in the above prices. 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.

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

  • Structural Engineer - to get through the building regulations stage you’ll need a structural engineer to make sure your project is strong, durable and safe. These can cost anywhere between £950-£2500.
  • CCTV Drainage Surveyor - as part of building regulations 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 dispute.
  • Approved inspector - these are alternatives to your local building authority. They work to the same standards, but can prioritise your project meaning the building regulations process is more streamlined. These typically cost between £900-£2000.


Both your construction labour and materials will be subject to this rate. However, there are some exceptions…

  • If you use local tradesmen, who aren’t VAT registered, you won’t have to pay 20% on their labour, though charges on materials will still apply.
  • Work to listed buildings is eligible for VAT relief.
  • Any residential property that has been vacant for two years is also subject to reduced VAT.

Other Costs

  • Local authority - your local authority will charge you for submitting planning applications and for gaining building regulations approval.
  • Warranty - side return extensions are a big investment, so they are well-worth protecting. A 10-year warranty policy will charge 2% of construction costs.
  • Insurance - we recommend reviewing your home and life insurance policies before any big project. It’s always better to be safe than sorry.
  • Contingency costs - If all goes well you’ll stick to budget, but we recommend setting 10% of what your construction costs aside, just in case anything unexpected pops up.

Financing your extension with Resi

By working with Resi, you’ll have access to our unique Finance service, which allows us to nail down your budget early on and help you to secure the best funding.

While many people look at short-term costs, don’t forget to consider the big picture. Unless you have a nice pot of savings, it’s likely you’ll be financing your new build through a loan. One of the best things you can do when choosing your provider is to go with a specialist lender, who has experience in handling property-related investments.

As official partners to the Mortgage Advice Bureau, our Finance service is able to source the best lenders and rates for your project, working with your designers to make sure you get the right amount of funding at the right time.

We’ll also help ensure your investment is properly protected by sorting out the best insurance policies for you. As the COVID-19 crisis illustrated, you want to make sure that no matter what happens in regards to your health or employment, you can still make your repayments.

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

Or for some side return extension inspiration, check out our portfolio page.

Side return extension cost

How much will it cost to build?

Which part of your property would you like to extend?

Ground Floor
First Floor


What is the average construction cost of a side return extension?

Construction costs can widely vary. On average, they can range from £28,000 to £52,000 for London, and £23,100 to £43,000 outside of London. However, these are only the construction costs. For the overall project costs you will also need to consider other factors such as architects costs, administration fees, surveyors, etc.

Can a side extension be double-storey?

A side extension can be single-storey or double-storey depending on your property and space available.

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