Many of London’s 19th and early 20th century properties have an original brick ‘extension’ to the rear. These are particularly common on terraced and semi-detached houses. A side return extension infills the space alongside the original ‘extension’ and is one of the most popular methods of improving and extending your home. This relatively small-scale alteration can transform a property.

How much does a side return extension cost in London? In this article, Architecture for London outlines estimated costs and provides example case studies for comparison.

View Architecture for London’s portfolio of recent residential projects.

What is a side return extension?

A side return extension is an infill of the passage to the rear and side of a property, commonly between two houses. While rear extensions often result in a loss of garden space, the side return infill makes use of a typically disused part of the property.

Side return extensions can transform the character of the ground floor with little effect on useable garden space.


How much does a side return cost in London?

It can be difficult for homeowners to estimate costs prior to the start of an extension project. Many online resources do not take into account VAT and professional fees (such as architects, engineers and party wall surveyors). Some only provide averages for the construction of the shell of the extension; essentially the roof, walls and floor. There are of course, the following significant sums to consider in addition to the shell:

  • Glazing, such as rooflights and doors
  • Finishes
  • Kitchens, WCs or utility rooms
  • Underfloor heating
  • Landscaping
  • Associated internal works to the existing building

Some articles on the subject estimate the shell cost of a side return extension between £40,000 and £60,000 excl. VAT, with additional costs of at least £85,000 for the fit out of the space.

In reality, it is very difficult to give a total figure for the cost of a ‘standard’ side return, as there are so many sizes and design options to consider. There are also difficulties in splitting the project into ‘shell’ and ‘fit out’ as items of work often overlap, leading to inaccuracies.

A more useful approach is to break the total estimate down into ‘per square metre’ rates, which will be defined by the project aspirations.


How much does a side return extension cost per square metre?

Because a side return extension is small in scale, some may assume the project will be low in cost. However, there are fewer economies of scale in smaller projects.

The cost per square metre for side return extensions is, therefore, comparatively high.


In this calculation, we will use four examples (A, B, C and D) with various specification options, based on Architecture for London’s completed project data. These include both shell costs and fit-out costs. The figures exclude VAT and consultants fees, however, as these are detailed in the following section.

A side return extension may cost £4,500 per square metre in London. A typical size for side return extensions is 10 or 11 sqm, so this equates to around £50,000 (A).

A side return project rarely stands in isolation though; an additional cost will apply for works to the adjoining room (usually the kitchen). This will likely require fitting new kitchen units and appliances, along with a reasonably intensive refurbishment, we may estimate a rate of £3,000 per square metre for the existing room. Assuming a size of 20 square metres, this results in an additional £60,000. So in total, this gives an estimate of £110,000 (B).

While the above figures allow for reasonably good quality kitchen units and appliances, there is significant scope for costs variation with these items. A branded kitchen from a high-quality supplier may result in an extra £30,000 cost in addition to the above figures, so in this case, the total estimate might be £140,000 (C).

Costs for glazing, external doors, and finishes will also be more dependant on the project specification and ambition. While reasonable quality items are allowed for in the above figures, if high-end options are required (polished concrete floors, large expanses of glass, large slimline sliding doors, a new cloakroom etc.) then you may wish to budget an additional £40,000. This may give a combined higher end cost of £180,000 (D).

Depending on the aspiration, the figures may range from £50,000 to £180,000 as shown in the examples above.


How much should I budget for VAT and fees?

We will now consider the additional sums for VAT and fees. The current applicable VAT rate is 20%. As a rough budget indication, we may estimate fees for all consultants (architect, engineer, party wall surveyor, quantity surveyor etc.) to be an additional 20% on a project at this scale. When all costs are taken into account, the total sums in the above examples are therefore approximately as follows:

  • A. £70,000
  • B. £155,000
  • C. £195,000
  • D. £250,000

From the description of each of the above projects you can find the option that most closely aligns with your own aspirations, and therefore understand if the figures in A, B, C or D are most relevant to your project.

If the size of your side return extension is much bigger or smaller than 10 sqm and a corresponding 20 sqm of existing space, you can adjust the numbers proportionally to obtain a more accurate estimate for your project. View our article on architect’s fees for further information.

Where can I make savings?

If your budget is tight, here are some ideas for low-cost side return extensions:

  • Avoid adding bathrooms, these can add £10,000 on to the cost.
  • Joinery is expensive, so minimise the number of kitchen and storage units.
  • Avoid a separate utility room or pantry.
  • Ask your architect to design a kitchen based on standard carcasses, rather than buying an expensive branded kitchen.
  • Avoid overly large glass windows and doors, as bespoke glass is expensive.
  • Ask your architect for suggestions on alternative finishes and fittings that are similar to the ones you like.
  • Consider phasing the project if there is an element that is easy to complete at a later date.

Architects are used to dealing with budgeting and will have project-specific suggestions of how to cut costs without being detrimental to the quality of the design.

London side return extension cost case studies:

This section outlines costs from three of Architecture for London’s completed projects. While they provide useful examples, we’d always recommend taking a project-specific approach when estimating your own sums. All figures below exclude VAT and consultants fees.

1. A house in south London (small scale and limited budget)

Single-storey side return extension with some refurbishment throughout. Kitchen and WC relocated. New underfloor heating to the extension. External patio.

  • Total contract sum: £120,000
  • Total works area: 90 sqm
  • Average cost per sqm: £1,300
  • Ratio Existing to New: 80:20
  • Added area: 16 sqm

2. A house in north London (small scale and medium budget)

Two storey extension to outrigger and single-storey side return extension to mid-range spec. Fully stripped back to brickwork. Roof fully replaced.

  • Total contract sum: £230,000
  • Total works area: 115 sqm
  • Average cost per sqm: £2,000
  • Ratio Existing to New: 80:20
  • Added area: 22 sqm

3. A house in north London (small scale and medium budget)

Single-storey wrap around extension to mid-range spec. 26 sqm to rear area excavated and underpinned to create additional ceiling height.

  • Total contract sum: £190,000
  • Total works area: 90 sqm
  • Average cost per sqm: £2,150
  • Ratio Existing to New: 75:25
  • Added area: 22 sqm


A side return extension can offer a spectacular transformation of your home. This can be achieved on a relatively low budget with a careful approach to design.

We would suggest using square metre rates to estimate costs at an early stage in the project. Once the design progresses, these figures can be refined with input from a quantity surveyor.

A quantity surveyor will be able to give more detailed information in a document of estimated costs, broken down into specific items.

If you are thinking of extending please contact us to discuss your aspirations and budget, or visit our projects page to see more of our previous work.