Basement Extension

A basement extension can be a smart way to increase the square footage of a property but it’s vital that the necessary works are fully planned and a comprehensive risk analysis is undertaken. Whether you’re converting an unused cellar or basement or excavating new space, it’s vital that the work carried out doesn’t negatively impact the structural integrity of your property or neighbouring buildings.

What Is Needed for a Basement Extension?

If you want to convert an existing cellar into a functional basement, you may not need planning permission to do so. This is because it constitutes a ‘change of use’. However, the exact nature of the extension and the scope of the works may mean that planning permission will be required, even if an existing cellar is present. Alternatively, if you plan to excavate a new space to facilitate a basement extension, then planning permission will be required.

However, planning permission isn’t the sole concern when you’re considering installing a basement extension. You may also require:

A Basement Impact assessment
A party wall agreement
A structural survey
Soil investigations
Tree surveys

In addition to this, all work will need to be inspected and certified to ensure it meets relevant building and safety standards.

Basement Extension London

Basement extensions are increasingly common in bustling cities where property owners are eager to maximise the value of their buildings. While you may have already extended a property upwards and outwards, a basement or under-garden extension provides an innovative way to access additional living space.

While basement extensions in London often highlight the success of these projects, there have been high-profile cases of things going wrong too. In some instances, homeowners have faced disastrous consequences due to poor planning or unexpected construction issues. By using these cases to inform your own plans, you can avoid the pitfalls other homeowners have faced and ensure that your basement extension provides a safe, functional and attractive living space.

Planning Permission Needed for Basement Extension

Whether planning permission is required for a basement extension depends on various factors, including the existing layout.

If excavation is required, planning permission will be needed but if the building already has a cellar and the extension will simply constitute a change of use, then you shouldn’t need planning permission. However, the features of your planned extension may require planning permission, even if you’re converting an existing cellar.

If you want to add an external entrance and/or lightwells, for example, then you’re likely to need planning permission. Similarly, if your plans constitute making structural changes to the building or the property is in a conservation area, you’re going to need planning permission.

Basement Extensions FAQs

Will I Need a Basement Impact Assessment?

Many councils now require a Basement Impact Assessment to be conducted as part of the planning application process. Councils such as Wandsworth Council provide guides for homeowners. However, even if a Basement Impact Assessment isn’t mandatory, it can be advantageous as it will evaluate the potential impact of the planned works.

Will I Need Building Regulations Approval?

Yes. The work undertaken will need to be inspected and certified, so be sure to factor this into your timeline and budget.

Will I Need a Party Wall Agreement?

If you share a wall with a neighbouring property and you plan on making structural changes, then it’s likely you’ll need a Party Wall Agreement. If you’ll be cutting holes into a shared wall to fit new beams or excavating within 3 metres of a shared wall, for example, then a Party Wall Agreement is essential.

Get Expert Advice Ahead of a Basement Extension

At TCL Chartered Surveyors, we’re on hand to help property owners, architects and neighbouring building owners navigate the complexities of basement extensions. Whether you’re planning your own extension or are concerned about proposed works near your property, we can help analyse the potential impact and determine what action should be taken to mitigate any risks and protect the integrity of the property.