London basement conversion: Your five-minute guide

London basement conversion: Your five-minute guide Conversions & Extensions

The average size of residential properties in the UK is among the smallest in the world. This also applies to luxury property.

Homes costing £5m or more in Fulham, Chelsea, Hammersmith, Balham, Battersea, Vauxhall, Westminster, Richmond, Kensington, and Mayfair are much smaller than leading cities, with the potential exception of Monaco.

In this article, we cover the benefits of basement conversions and explain the role an architect has through the entire process from initial concept to final sign-off.

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What is the difference between cellar conversions, basement conversions and basement extensions?

While all three terms are used interchangeably, there are differences. The definitions are:

  1. Cellar conversion: the renovation of an existing cellar, typically used for storage space, into a functional living area.
  2. Basement conversion: the renovation of an existing basement that may already be partially or fully furnished into a usable living area. Basements are usually larger than cellars.
  3. Basement extension: the creation of a new basement level beneath an existing property. Sometimes on these projects, an existing basement is entirely replaced.

In this article, we’re covering basement extensions only. However, as they are more commonly referred to as basement conversions, we refer to them by their popular term instead from this point on.

Why basement conversions are popular

The main reasons why homeowners want a basement conversion for their London home are:

  • More living space: You can add significant extra square footage to your property and use it in any way you choose, ideal for growing families.
  • Increased property value: Homes with large basements in London are scarce. A basement could add 25% or more to the value of your property.
  • Noise reduction: Basements have superior sound insulation properties making them ideal for home cinemas and music rooms. That noise-proofing can also make them the quietest space in a house.
  • Additional storage: Basement conversions create more storage space as well as additional living space. Use this extra space to declutter your property, storing items like occasionally used tools down there.
  • Separate living space: Many properties that have had basement conversions in London use that extra space for separate but attached dwellings like a granny flat.

Why loft conversions, a past favourite, don't deliver any more

What other choices do you have? You could:

  • Convert your loft: They’re great if you have the headspace but loft conversions offer particular aesthetic challenges, particularly on listed buildings.
  • Extend your property: How many London luxury properties occupy the plots they’re located on makes building quality extra space difficult.
  • Move home: There is a significant disruption to family life when you move home. It’s expensive too. If you buy a £10m home in London as your primary property, that costs £1.1m in stamp duty (£1.41m if bought as an additional property). The cost of a basement conversion project may be less than the stamp duty and additional fees you pay if you choose to move home.

London basement conversion facts and figures

All of these three alternatives are viable options. But basement conversions are the popular choice.

According to a study by Newcastle University’s School of Architecture, Planning & Landscape, London basement extensions are now as normal as loft conversions. Researchers discovered that the City of London and its 32 boroughs approved 7,328 basements between 2008 and 2019.

Statistics revealed that 5,813 of the basement conversions were single-storey conversions, 1,344 were large basements (big enough for swimming pools) and 171 were mega basements, some extended under the garden going as deep as three storeys.

Hammersmith and Fulham had the highest number of standard basement conversions whereas the Royal Borough of Kensington and Chelsea has the most large and mega-basement conversions.


Roger Burrows, professor of cities at Newcastle University, speaking to the Guardian newspaper, commented that “what we’ve seen is a normalisation of single-storey basements as the 21st-century version of the loft conversion”.

The schemes examined included:

  • 532 swimming pools
  • 814 cinemas
  • 1,695 gyms
  • 689 wine cellars
  • 607 games rooms
  • 342 steam rooms or saunas
  • 154 staff quarters

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What an architect does at all stages of the basement conversion process

Your architect and construction team are arguably the two most important parties on your London basement conversion project.

Below, we list the three main stages of work and what your architect will do for you.

1. Laying the groundwork

The architect is the person who’s responsible for interpreting the vision you have for your basement project.

The process begins with a visit to your home. They’ll assess the existing structure, building and site conditions.

They look for issues affecting access or drainage, review soil conditions and take measurements.

They also identify any constraints or opportunities that might impact the design, particularly important for construction under an existing building.

London basement conversion: Your five-minute guide

2. Working with the neighbours

Your architect will identify when party wall agreements may be needed. These agreements cover the walls, boundaries and excavations (the so-called “party walls”) you share with neighbouring properties.

They’ll provide support and strategy during your negotiations with neighbours so that you can come to a party wall agreement you and they are happy with.

3. Designing your basement conversion

The job of your architect is to develop a functional, practical and aesthetically pleasing design that matches your budget, preferences and needs.

Their design must take into account the potential challenges of a basement conversion project, Building Regulations and structural requirements.

From the information you share with your architect about what you want from your basement conversion and what they learned from their site visit, they begin the design process.

As the process moves along, you’ll get revised drafts that take into account your feedback. Over a short space of time as the revisions become less significant, your architect will begin to send you more detailed plans including 3D models so you can see what the final floor and room plan will look like.

4. Taking care of the finer details

As the plans and 3D renders start to come to life, this is the time when you choose the materials you use in the interior of your basement conversion.

Your architect will make sure that all aspects related to materials, finishes and fixtures fit within budget constraints including the cost of maintenance.

5. Design and budget sign-off

Your architect will stick to your budget throughout the planning process.

They’ll often advise you when a choice or alteration you might make may push it beyond the budget. Likewise, if a choice or alteration saves money, they’ll let you know giving you an opportunity for further personalisation.

Once the final design is agreed upon, you’ll be advised of preliminary cost estimations based on your architect’s best estimate of the cost of labour and materials for the work.

6. Going for planning permission approval

From the very start of the project, your architect will consult with your local authority on your project. They’ll be there to field questions from planning officials and provide additional information when requested.

They’ll first aim to establish whether you require planning permission or you can claim the project falls within permitted development rights.

London basement conversion: Your five-minute guide

What if planning permission is required?

They’ll prepare everything needed for the process including site plans, sections, elevations and more. If a project requires a design and access statement, your architect will create that outlining the design principles and concepts that apply to the project.

Depending on the process, they may need to consult with other professionals including a structural engineer on issues like flooding and tree surveys.

What if planning permission is granted with consent?

Sometimes, consent for a basement conversion is provided subject to conditions.

Your architect will help you address those requirements and submit revised applications to the planning authorities.

What is planning permission is refused?

Where planning permission is refused, your architect can appeal the decision, making changes to the project with your consent so that they can alter the proposal to improve the chance of approval.

7. Help choose your construction team and brief them on the work

Before the build begins, your architect can help you choose which basement contractors do the work.

Many will have a list of preferred basement contractors, companies they’re confident in recommending because they’ve worked with previously.

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8. Liaising with the project manager when work is underway

Many clients ask their architects to liaise with the contractors throughout the process.

This is so that they can verify that work is being completed as agreed and to the required quality.

They also perform regular site visits to check on progress, quality and to resolve any problems that come up with the dedicated project manager from the construction firm.

During the build, your architect will also check that the project is compliant with Building Codes, all relevant legislation and the specifications outlined to the planning authorities.

9. Managing mid-project course changes

On many projects, specifications may have to change if there are unforeseen difficulties in a particular aspect of the work.

When this happens, your architect can approve the change so long as it doesn’t significantly change the final outcome. Where it does, they will consult with you to get your advice on how to proceed.

10. Sign-off and snagging

You can also ask your architect to inspect the work carried to ensure that it meets the required design and specifications when your contractor is asking for final sign-off.

They can also create snagging lists of outstanding defects or items for contractors to remedy.

London basement conversion FAQ

What does a basement extension in London cost?

It depends on the complexity of the work but you should budget between £2,500 and £4,000 plus VAT for the digging out of the space, the creation of the watertight concrete box and associated tasks like concrete flooring, roof slabs, water drainage, underpinning and steel work.

How much value does a basement add?

Estimates vary but a basement could add up to between 25% and 70% onto the value of your home depending on its size, internal layout and finishing.

How long does a basement conversion take?

The time it takes to complete basement conversions in London varies from a few weeks for a simple cellar conversion to several months for so-called mega-basements.

Can you live in a house while building a basement?

Yes, you can live in a house while building a basement but be prepared for lots of disruption, noise and dust.

Be prepared to spend more to relocate electricity, gas and water routing through your property while work is underway. Living in the property may also delay the speed of progress of the work.

Think carefully before you make a decision on the issue and speak with your architect and building contractor.

Can you extend an existing basement?

You can extend existing basements to create extra rooms like a gym, office, home cinema or extra bedrooms.

What stages are involved in the process of converting a basement?

There are two parts to the basement construction process: the shell-and-core and the fit-out.

The shell-and-core phase is when the basement excavation happens. Structural support is then added as well as waterproofing to keep the area comfortable and dry.

The fit-out is where the space is turned into a living functional area.

Basement conversion London specialists

If you want to add space to your existing property by building a new or extending an existing basement, contact WindsorPatania.

Working from offices in London, Cambridge and Milan, you’ll have single point of contact throughout your entire project – an architect whose experience, skill and passion ensures that we deliver you the outcome you want.



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