Can I add a basement to my property?

Homeowners continue to add basements to their property, often because it is a cost-effective alternative to buying a bigger house, and options for building higher or outward might be limited.

The process of constructing a basement is actually quite straightforward – underpin the existing walls (including any party walls shared with neighbours), dig out the hole, cast a floor slab and fit out the space (oh, and make sure it is properly waterproofed). But, a basement project also comes with a bundle of potential difficulties for the unprepared. It’s a major project, usually expensive, and must be designed and managed properly.

Always use basement experts. Do not attempt a ‘do it yourself’ basement. Several years ago, we were called to a house where the neighbours had excavated their back garden for the basement. They hadn’t thought about building regulations or temporary support of the earth walls, let alone the health and safety of the cheap labourers who were hand-digging a huge (4m x 4m) hole, 3m deep. When the problems were realised, the cheap DIY option had cost them an additional £25k.

You must ensure you use a reputable firm for the construction who are familiar with basement design and construction.  Find out about their previous projects and get recommendations.

As the enthusiasm for basements under terraced houses develops, so do the basement construction ‘specialists and experts’ who incorrectly see the works as straightforward and a bit like a loft conversion, only downwards. Lucrative basement construction seems like a good idea, especially if you can do it cheaper than others by cutting one or two little corners – like underpinning properly. In the worst case we have been involved in, the basement constructor ended up paying £250,000 in fees and costs to rebuild his neighbours’ house because the engineering design was not properly executed.

Image inside basement undergoing renovation

How do I plan a basement building project?

Employ good chartered architects and chartered structural engineers who are used to working together. They will be able to interpret what you want and how it can be constructed. Importantly, they will also know what you will not be able to achieve. You may also need a planning consultant if the planning consent is going to be difficult – and many local authorities now have restrictive policies for some basement constructions.

Do I need a party wall surveyor?

You will need Party Wall Agreement for your basement unless you live in a detached property and all excavations are more than three metres away from any other property. That means you will need to engage a party wall surveyor – make sure you use someone who is familiar and experienced with basements.

Why is party wall agreement important for basement projects?

Well, digging a great big hole next to another house, never mind beneath your own, needs to be designed and done properly because occasionally things like living rooms fall into it! The underpinning of existing walls must be spot on and properly designed.

In our opinion, it is always the temporary work that causes the problem – that piece of temporary support that is needed between having a basement and not having a basement.  And it is this that is often under designed, or left to the contractor to sort out.

We would go so far as to say that every basement project where we have provided party wall services has ended up with damage (usually minor) being caused to next door, even where the schemes were properly designed, managed and constructed. The work is invasive and inevitably settlement happens. This BBC Inside Out extract shows the sort of damage that occurs.

What about insurance?

Decent insurance is a must.  Make sure your designers, architects and engineers have appropriate Professional Indemnity Insurance and that your contractors have satisfactory liability insurances – remember if this goes wrong you may be liable in the first instance.

Your contractor’s insurance may well not be good enough and you (not the contractor) will be personally liable for the costs of any damage – although you may well be able to claim against them. Crucially, in the first instance, ensure you have a trusted and well-respected builder undertake the work.

You might also ask your design team to provide a Professional Consultants Certificate when the job is complete to confirm that they inspected it regularly during the work and it was built to specification.

Do I need planning permission for my basement?


Do I need building regulation approval for my basement?


What if my neighbour is building a basement?

If you’re next door to a basement construction, always, always get decent party wall advice.

Useful links