How hard is it to do a basement conversion?

You may have heard about some of the very extravagant basement conversions currently taking place in central London. We’re talking four floors, swimming pools, gyms and saunas. When you’ve reached the limit of adding space on top, behind and beside a property the only other option is below.

It is worth noting that basement conversions cannot be done on all properties, consult a structural engineer and architect for specific advice.

Basement conversions are a great way to add more space to a property, especially if rear extensions or loft conversions are not feasible (or have already been done). There are two main types of conversions 1) turning an existing cellar into a habitable space and 2) excavating a new basement – the latter is the more complicated and expensive.

Planning Permission for Basement Conversion

If you are turning an existing cellar into a habitable basement then it is very unlikely that you will need any planning permission - so long as the new conversion is not then used as a separate property. Remember, if there are going to be any changes to the outside of the property then planning permission may very well be required.

Excavating a new basement will most probably require planning permission as well as the need for a Party Wall Notice between you and your neighbour(s). we strongly advise that find a Party Wall Surveyor and consult about your specific works.

The good news is that converting an existing cellar 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. Even under PD, it is a good idea to apply for a certificate of lawful development from your local authority; doing so will give you the relevant certification to prove that your scheme met requirements and did not need planning permission, this is particularly helpful when obtaining mortgages or refinancing your property.

Building Regulations for Basement Conversions

Ensuring your basement conversion is “good and proper” is essential, especially as it could affect not just your own house but your neighbours’ too. To make sure the conversion is legally useable or habitable it must meet Building Regulations. In order to achieve this the conversion will have to meet certain criteria, such as sufficient levels of insulation, creating emergency escape routes and having a minimum head height.

All the structural work will need to be passed by a Building Inspector, to ensure that it will not impact the rest of your house or your neighbours’ properties.

Party Wall Agreements for Basement Conversions

Just like a loft conversion or rear extension, a basement conversion will require the owner to serve Party Wall Act Notice to Adjoining Owners.

If you live in a detached house, you will not need to have Party Wall Notices in place unless the excavation is within 6 meters of the neighbour’s foundation. But, if you live in a terrace, end of terrace or semi-detached property then you 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. This has to be done in accordance with the Party Wall Act.

As the property owner you will need to appoint a Party Wall Surveyor and remember that each of your neighbours have the right to appoint their own – you are liable to pay for all their costs.

With your neighbours onboard, planning permission in place and building inspector instructed you are ready to crack on with your project. More articles around basement conversions will be published soon – keep an eye out for them.


As you can see a basement conversion doesn’t necessarily have to be hard or any more complicated than other conversions. If you are looking to do a basement conversion and you need to find a Party Wall Surveyor in London then contact Seven One Associates to get help and support on how the works will be affected by the Party Wall Act.

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