Basement conversions: the ultimate guide to converting a basement

A basement conversion might be the best way to add space and value to your home if you don’t have the room to extend above ground. This three page feature tells you all you need to know about how to convert a basement

Basement conversion: kitchen diner and living space in a basement extension to a Victorian home with a golden retriever
(Image credit: Simon Maxwell)

Planning a basement conversion is so much more complicated than planning, for example, a single storey extension. However, much of the planning will be done for you by specialists since, unlike simpler extensions, this is not a project that you can easily run yourself. Use these guidelines below to ensure you've got all the planning that you can practically do yourself sorted in advance.

Does a basement conversion need planning permission?

Converting an existing residential cellar or basement into a living space is unlikely to require planning permission. This is provided it’s not going to be used as a separate property, and that the external appearance of the original house is not significantly altered.

Major works to excavate a new basement, adding a separate unit of accommodation, and/or altering the external appearance of your house, are likely to require planning permission, even if you’re converting or expanding on an existing cellar space. 

In all circumstances, you should contact your local planning authority for guidance before starting any work. You can apply for planning consent yourself, or employ a professional architect or specialist basement company to apply on your behalf. They will have an insight into local planning regulations and projects in the area that have been successful.

Adding or converting a basement could be the ideal way to create space if you have a garden that is too small to extend into. This is exactly what the owners of this Grade II-Listed Georgian house did

How deep will a basement conversion need to be?

How deep your basement conversion needs to be depends, to a large degree, how you're going to use it. An intimate cinema room or small home gym or utility area will get away with a lower ceiling height than a living space that you want to feel part of the original house. 

For the latter use, expect to have to dig deep (around 4m to 4.5m) to get a high ceiling (around 2.7m to 3m). This will also allow you to put in light wells and roof lights so that you get an airy, bright feeling room, despite it being at basement level.

Converting a basement under permitted development rights

Converting an existing basement from, for example, a storage area into a habitable room can be completed under permitted development (PD) rights, unless you live in a Conservation Area or if your home is listed).

If working under PD, you may want to apply for a certificate of lawful development from your local authority; doing so will give you the paperwork to prove that your scheme met requirements and did not need planning permission.

The general PD criteria is on the government's planning portal, but check with your local council before you proceed, because some areas have more restricted rights.

Building regulations for basements

To ensure your new basement is legally habitable, it will have to meet building regulations by achieving sufficient levels of insulation, creating emergency escape routes and having a minimum head height, which may require structural supports, such as steel beams. Any structural work you carry out will need to be passed by a building inspector, to make sure that it won’t impact on the rest of your house or your neighbours’ properties.

Party wall agreements for basement conversions

The Party Wall Act provides a legal framework for preventing and resolving disputes in relation to boundary walls, party walls and excavations that are near neighbouring buildings.

Unless you live in a detached house, you will need to reach an agreement with your neighbours over shared walls before converting your basement. The correct notices will need to be issued and consents obtained from all neighbours whose boundaries might be affected at least two months before work begins.

If you are using a contractor to design, manage and complete your basement project, they will usually deal with any party wall agreements that need to be reached, along with planning applications and building regulations. If not, you will need to instruct a surveyor; your neighbour also has the right to appoint their own surveyor, and you will be obliged to pay for both surveyor fees. 

If you get your neighbours on board, they may consent to the work and there won’t be any fees involved, but you’ll need to get this in writing on a waiver form.

Tanking the basement

Tanking is the most common way of waterproofing an underground space. This involves applying a coating to the interior of the porous basement walls to create a waterproof barrier. This can be in the form of a membrane that is fixed to the walls or as some kind of waterproof render or sealant.

The two main methods to choose from are brush-applied tanking or a cavity membrane drainage system. Whichever you choose, products should be British Board of Agrément certified and you should receive an insurance-backed guarantee. Of course, adequate heating and ventilation will also help prevent condensation from forming.

Find out more about tanking a basement.

Preventing damp in a basement

Building regulations require that all floors and walls below ground level are waterproofed, to stop damp entering a building’s structure. Water penetration can be tricky to control; it’s not unusual to find basements where two or three different systems have been applied – bitumen, cellar paint and/or render – but all have failed for one reason or another. Find the right solution for your basement with an assessment by a waterproofing contractor with a proven track record.

For basements where the water table is higher than usual, or in old homes where the cellar walls may be more porous, you will need to have a cavity membrane drainage system. This takes water out of the basement, either naturally or with an electric pump.

The owners of this 1970s townhouse dug down five metres to add space with a basement conversion. Image: Ensoul

Finding professionals for a basement conversion

A specialist basement conversion company will undoubtedly do the best job in converting your basement; and a local basement conversion company will not only be familiar with the local council's foibles, but also with factors such as soil types, so appointing a local specialist should make the whole process smoother.  

As with any other building job, recommendation from friends and neighbours is your best bet, but you can also follow up members of trade bodies such as the Federation of Master Builders to find reliable contractors.

How long does a basement conversion take?

Revamping an existing cellar, involving applying a membrane lining system, digging out the sump and fitting a pumping system will take just a few weeks.

A full basement conversion, involving underpinning of the existing house will take several months.

Live in or move out during a basement conversion?

This will depend very much on access: if there is access directly into your garden or the road outside for soil to be removed, and the existing ground floor of your home is constructed from suspended timber, it's likely that you can continue to live there while the basement is converted. 

If, however, your home has a concrete ground floor which has to be removed and rebuilt, you will almost certainly have to/want to move out.

Bear in mind that larger firms will be quicker but possibly more expensive; smaller firms will be less flexible but possibly cheaper in the long run – however the conversion might take longer with them.

Ensure your builder has all the relevant warranties in place, and draw up a watertight contract using a template, such as a JCT homeowner contract. Ensure you are explicit within the contract about when payments will be made: payment upon completion of specific parts of work rather than weekly, for example, is a better route to take. 

You can find architectural technologists on ciat.org.uk and architects via architects-register.org.uk or RIBA. Read more in our guide to working with architects.

Designing a basement conversion >>

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