London Basement Conversion: A Quick Guide

The average size of residential properties in the UK is among the smallest globally, and this trend extends to luxury homes. Even properties valued at £5 million or more in areas like Fulham, Chelsea, Hammersmith, Balham, Battersea, Vauxhall, Westminster, Richmond, Kensington, and Mayfair tend to be smaller than those in leading global cities—Monaco being a notable exception.  

This article explores the advantages of basement conversions and outlines the architect’s role throughout the process, from initial design to final approval.

Basement Conversion Types

While the terms cellar conversions, basement conversions, and basement extensions are often used interchangeably, they refer to distinct types of projects. Here’s a breakdown of the differences:

1. Cellar Conversion: This involves transforming an existing cellar, often used for storage, into a livable space.

2. Basement Conversion: This refers to converting a basement that may already be partially or fully finished into a functional living area. Basements are generally larger than cellars.

3. Basement Extension: This is the process of adding a new basement level beneath an existing property. In some cases, a pre-existing basement may be completely replaced during this process.

This article focuses specifically on basement extensions, though for simplicity, we will refer to them as basement conversions throughout.

Why basement conversions are popular

There are several key reasons why homeowners in London are opting for basement conversions:

• Extra Living Space: A basement conversion allows you to add substantial square footage to your home, offering flexible space that can accommodate growing families or changing needs

• Boosted Property Value: Large basements are a rare feature in London homes. Adding one can significantly increase your property’s value, with some estimates suggesting a potential increase of 25% or more.

• More Storage Options: A converted basement not only adds living space but also provides additional storage, helping to keep your home organized by storing items like seasonal decorations or tools.

Basement conversions in London – facts and figures

All three options are valid, but basement conversions are by far the most popular choice. A study conducted by Newcastle University’s School of Architecture, Planning & Landscape found that basement extensions in London have become as commonplace as loft conversions. The research revealed that between 2008 and 2019, 7,328 basement applications were approved across the 32 boroughs of London and the City of London.

Of these, 5,813 were single-storey conversions, 1,344 were larger basements (often large enough to house swimming pools), and 171 were expansive “mega-basements,” some extending beneath gardens and reaching depths of up to three stories. Hammersmith and Fulham had the highest number of standard basement conversions, while the Royal Borough of Kensington and Chelsea led in large and mega-basement projects.

Roger Burrows, Professor of Cities at Newcastle University, commented in an interview with The Guardian that “single-storey basements have become the modern equivalent of loft conversions in the 21st century.”

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

Basement Construction FAQs

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.

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