How Much Does A Side Return Extension Cost In London

Amos Goldreich Architecture has carried out several side return extensions in London over the past decade. Each project is different, with different constraints and different ambitions. How much does a side return extension cost in London? Read on to find out.

Side return extension to Almington Street, showing new connection between old and new

What is a Side Return Extension?

A side return is the narrow strip of land that runs alongside the ground floor of a property. A side return is common in London due to the standard layout of Victorian houses. These are particularly common on terraced and semi-detached houses. The side return allows inner rooms of the property to still benefit from natural light.

A side return extension fills the gap between the existing property and your neighbour’s boundary. It is a popular way of extending an existing Victorian terrace. Transforming an underutilised area of the garden to create an open and bright extension to your home.

Pre-construction costs

It is important to understand there are more costs to consider than just the bricks and mortar of the extension itself. Consider the project costs in their entirety, including professional fees, local authority fees, and VAT.

Professional Fees

Architects – £12,000-£15,000: Architects bring knowledge, experience, and will help take the stress out of your project. They open your eyes to new ideas and will help to add value. We would always recommend appointing an architect: How to Choose an Architect.

Architectural fees can vary. When approaching architectural practices, always ask for a full breakdown of the scope of works.

Ways an architectural practice could present their fees:

  • Percentage of the construction cost
  • Lump sum or fixed fee
  • An hourly rate

A lump sum fixed fee always offers more cost certainty as the fees are capped to a fixed fee.

Structural Engineers – £1,500-£2,500: It is likely structural changes are needed to accommodate the new side return extension. Structural calculations will need to be submitted as part of the Building Control Application. A good structural engineer can spot opportunities within the existing building fabric and can offer create solutions that can save money.

Other professional fees

Party Wall Surveyors – With a typical London side return extension, you likely to be building within 3 metres of your neighbour’s boundary. Therefore, 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.

Local Authority Application Fees – your local authority will charge you for submitting planning applications and for gaining building regulations’ approval. Depending on your property and its location, you might be able to carry out the works under Permitted Development Rights. These rights may not apply if you live in a:

  • Conservation Area
  • An Area of Outstanding Natural Beauty
  • An area with an Article 4 Direction in place
  • A listed property

Always consult local planning policy and ask an architect or planning consultant to advise.

Approved Building Control Inspector – these are alternatives to your local building authority. Their processes are streamlined which can save time on your project. Fees typically cost between £900-£2000.

Specialist Consultants: Some specialists may be required depending on the nature of the project. These could include:

  • Landscape designer
  • Interior designer
  • Environmental consultant,
  • MEP services consultant
  • Planning consultant

Further Considerations

Contingency costs – Period properties in London always throw-up unusual and ad hoc construction techniques that are only uncovered once opening-up works commence. Always allow a contingency for unforeseen issues that arise during the construction phase.

Insurance – we recommend reviewing your home insurance policies before any construction project starts. We recommend “Renovation and Extension insurance” being taken out at the outset of the project.

Re-arranged kitchen at Tier House project to create a bright space

Building Construction Costs

No two London side return extensions are the same. Properties vary, house plans vary, and client’s ambitions differ from project to project. This makes it difficult to produce a one size fits all price.

To understand the build costs involved, you should first assess the condition of your existing property and create your own list wish. The following factors can all have dramatic effect on cost:

Ask Yourself these 8 questions:

  • Is the side return extension part of a wider renovation project including decoration throughout? The economies of scale involved in small-scale projects mean you will achieve better value if you also carry out a full renovation of your home.
  • Will structural changes need to be made to achieve the desired extension? Often to make the side extension feel integrated into the existing house, reconfiguration of the existing building fabric will be needed. This can include adding new doors or rearranging internal walls.
  • Does the existing house need an upgrade? Are you experiencing any damp? Do you find your house loses heat easily and needs a thermal upgrade? Does the house need a re-wire? If your budget allows, it is sensible to include all the above.
  • Are you looking for bespoke design solutions to add a wow-factor? Anything bespoke will add cost but will add value.
  • Will your kitchen or any utilities need to be moved? Relocation of services and plumbing will add costs but might be fundamental to achieving your desired living arrangement.
  • Will the position of internal doors need to change to achieve your layout? Or will new doors need to be added?
  • Will you need to re-landscape the garden as part of the works?
  • Will you be moving out for the duration of the works? It is likely this will speed up construction as the builders will not to work around you. However, unless you are able to move-in with relatives, the cost of a rental property needs to be considered.
New openings at House for a Gardener connecting the house to the garden

Use our construction cost calculator

We have developed a construction cost calculator for residential projects, updated for 2023 price per square metre build cost prices for London and the UK. The cost calculator is to be used as a guide only. It is always best to plan and engage with a well-established residential architecture company.

How much does a side return cost per square metre?

As every project is different, a more reliable way to estimate the cost of a project is to apply a Cost per M2.

In London, a side return extension can cost anywhere between £3,500 – £5,000 per square metre for the “bricks and mortar” alone.

This means a 10m2 extension to will range from £35,000-£50,000.

In London, we estimate a reasonable refurbishment cost for an existing room will be around £2000-£3500 per square metre.

Contractors and builders will have a minimum start-up cost. This will include labour, skips, on site facilities and administration costs. As a side extension is relatively small, the cost per M2 is more expensive than a complete new build house for example.


New build projects are not subject VAT. Although adapting existing housing stock is a much more sustainable form of development, extension and renovation are subject to 20% VAT. It is important to clarify if VAT is included in any quoted prices.

How to maximise value

  • List your priorities in order. Agree where you are willing to spend extra to guarantee what you want.
  • This could be better connection to the garden, a more open-plan home or a kitchen that better suits your needs.
  • Small adjustments, such as new or changed internal door positions can make a dramatic change to your space
  • Invest in energy upgrades as you will see cost benefits in the long run.
  • Avoid moving kitchen locations or services and plumbing to minimise cost.
  • Maximise any storage opportunities.
Tier house project as viewed from the rear of the property showing side return extension

How much does a side return extension cost in London?

Case study Projects

Almington Street: Side return extension in north London with ground floor refurbishment only

Single-storey side return extension with some refurbishment throughout the ground floor only. Structural changes to achieve new access. Kitchen relocated. New underfloor heating to the extension and new WC. Small external patio and garden works. See a detailed case study here.

  • Total contract sum: £100,000
  • Total Client Supply Items: £50,000
  • Added extension area: 13 sqm
  • Refurbishment area: 50 sqm
  • Average cost per sqm: £2,300

House for a gardenerSide return and rear extension in north London with refurbishment throughout

Single-storey side return extension with a rear extension plus full refurbishment throughout. Structural changes to achieve new access. Kitchen relocated. New underfloor heating to the extension and new WC. External patio and extensive garden works.

  • Total contract sum: £190,000
  • Total Client Supply Items: £60,000
  • Added extension area: 20 sqm
  • Refurbishment area: 150 sqm
  • Average cost per sqm: £1,500

Tier HouseSide return extension in north London with extensive refurbishment throughout

Single-storey side return extension with some refurbishment throughout. Structural changes to achieve new access. Kitchen relocated. New underfloor heating to the extension and new WC. External patio and garden works.

  • Total contract sum: £220,000
  • Total Client Supply Items: £50,000
  • Added extension area: 10 sqm
  • Refurbishment area: 155 sqm
  • Average cost per sqm: £1,700

Depending on the ambition and type of project, the cost may range from £100,000 to £190,000 as shown in the examples above.

Ready to start your project?

New skylights and openings create a bright and spacious side return extension

Amos Goldreich

Amos Goldreich Architecture is an award-winning architecture practice led by Amos Goldreich. We deliver full architectural services in close collaboration with our clients and end users, to create buildings and places which have a positive impact on people’s lives and are a joy to experience. For us, architecture is about people and so our work is always delivered with an acute sense of empathy and pragmatism. Much of our work has a strong humanitarian focus – we are always aware of the positive and progressive values that buildings can deliver to people and communities.

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