Unlike in the US where basements are ubiquitous, coming across a property with one in the UK is somewhat of a rarity.

Granted, we still have a number of older buildings in the UK with basements or cellars – often dating from the 18^th and 19^th centuries – but even these were only intended as austere servants quarters or cellars for storage. 

In recent years, an increasing number of UK homeowners have been waking up to the subterranean potential of their properties. It’s easy to see why; according to Savills, basement conversions can add between 10-15% to your property’s value, while the Home Owners Alliance claims it can add as much as 20%!  

In order to help you make the right decision, we have created this basement guide to answer all of your pressing conversion and extension questions.

Basement conversions in London

While basement conversions can be a great idea no matter where you are in the UK, as one of the most heavily populated cities in the world, creating extra space with a basement conversion can be particularly worthwhile in London. 

In fact, basement conversions in London have experienced a boom of late. This is especially the case in more affluent areas of the city where space is a premium and building regulations are strict yet residents have the budget for extensive building projects. Some London residents have been able to extend their already capacious properties with three or even four-storey basements.   

Basement extensions are so popular in the capital that there have been notable cases in the media where high-profile celebrities have clashed over basement expansion work. The popularity of these ambitious projects has even led to the coining of a new term for properties that have extensive underground space – ’iceberg homes’.

Common reasons for basement conversions:

The principal benefit of basement extensions is that they can be used to add more living space to a property (or even a separate flat) but they are increasingly being converted for more imaginative reasons. These include:

• Gyms
• Swimming pools
• Cinema rooms
• Games room
• Wine cellars
• Recording studios
• Underground parking
• And many, many more.

Do basement extensions add value to your property?

A basement conversion can be an effective way to make the most of your current space. However, they’re not for everyone. It all depends on your property, your budget and your ambitions.  

For example, converting an already existing cellar is going to be significantly cheaper and easier than excavating a basement from scratch.

While enjoying the newly created space may be a secondary benefit to adding value to your home, it’s important to remember that there is a ‘ceiling value’ to most properties. This value can be near impossible to surpass, no matter how extravagantly your new basement conversion may be.

Whatever your plans and whatever your budget, it’s crucial that you employ the expertise of local estate agents when assessing the viability of investing in a basement conversion. (Enter your postcode here to find the best local estate agents in your area). 

How much does a basement conversion cost?

In general, basement extensions are far cheaper when no excavation is required – either to create the underground space from scratch or to extend the headroom of an existing basement or cellar. Cellar conversions roughly cost half as much as building a completely new basement.  

If you have no existing underground space and you’re creating a basement entirely from scratch, you can expect to pay between £3,000 – £4,000 per square metre. A large portion of the costs relate to the necessary groundwork: this includes excavation and underpinning the property’s foundations, as well as plumbing, lighting, damp-proofing and ventilation. 

Other factors that may contribute to basement conversion costs include:

• Incorporating an external access  
• Architect/surveyor fees
• Planning permission application fees
• Building regulations application
• Party Wall agreement

Do you need planning permission for a basement conversion?

As planning rules vary depending on your location and are often changing, it’s recommended that you contact your Local Planning Authority regarding your basement plans before any work begins.

Cellar conversions that do not require any changes to the external appearance of your property are unlikely to need planning permission.

If your basement conversion requires excavation and changes to the exterior of your property (like adding a light well, for example) then you are likely to need planning permission from your local authority.

Basement Impact Assessment

Due to the growing popularity of basement conversions and extensions, many councils (especially in London) now require the submission of a Basement Impact Assessment as part of the planning application process.

Providing the necessary information for a Basement Impact Assessment can be costly and typically requires the assistance of a property surveyor. You should budget for this cost and discuss it with an experienced estate agent before starting your basement conversion project. 

If you’re thinking about embarking on a basement conversion project it’s crucial that you discuss your plans with a knowledgeable local estate agent first. They will be able to advise you on the viability of your investment, as well as guide you through all the necessary legal requirements. Helmsmen can help you find the best local estate agent to ensure your basement conversion goes as smoothly as possible.