Building an extension of any kind is no easy task, but building a basement onto your home is particularly challenging – even more so in London where space is tight and chances are, you’ll have neighbours nearby. 

If you have arrived at this blog post then you’re probably in the very early stages of building your extension. Perhaps you’re considering submitting a planning application or inviting builders to quote once you have done a little online research into the potential costs of building a basement. If so, then you’re in the right place. 

A completed basement can cost anywhere between £3,000-£5,000+ per square meter, but with so many factors that can influence the cost of your extension, from the size you have in mind to the ground conditions, waterproofing requirements and more, this is nothing more than a ballpark figure. In this blog post we’ll run through some of the key items that are involved in building an extension which will all need to be budgeted for. 

1. Insurance

With a basement extension it’s important to consider insurance right from the get-go. This is no ordinary extension – you’ll be excavating the ground from under your home, and moving so much ground can have a significant impact on both your own home and your neighbour’s homes. 

One London homeowner and neighbour of someone undertaking a basement extension commented on a public forum under the name of MrsOctober

“[A basement extension company] has recently finished a dig next door to us and our house (when it stops moving!) is going to need a whole load of repairs, redecorating and even a new front door. If you’re [in] a terraced house you must factor in those costs when you’re doing your budget.”

Whether you live in a detached, semi-detached or terraced house, securing planning permission for a basement doesn’t mean that damage to your property and your neighbour’s properties won’t occur – and your usual home insurance is not sufficient for this kind of undertaking. For a basement extension, you need specialist cover. MrsOctober is right to point out that you are responsible for the impact your extension has on neighbouring properties – but the cost of these repairs does not have to come out of your own pocket if you have the right insurance in place. 

At Ellis David, we are specialist insurance brokers and can offer both impartial and expert advice to make sure that you have the right insurance in place for your basement extension project. Get in touch with our team today to find out more.

2. Architect fees 

Architect fees don’t come cheap and can account for between 9% and 15% of the overall build cost – so these are worth bearing in mind when you are scoping out the costs for your project.

3. Other fees 

A whole host of other professional fees that need to be accounted for include:

  • Structural engineer fees 
  • Your planning application (if needed)
  • Building Regulations approval 
  • Party wall agreements (which can cost up to £750 per neighbour)
  • Survey fees for drains, water table, trees and general geology.

4. VAT 

Many of the professionals you are contracting on the job will also charge you VAT at 20% and so this is another cost that should be budgeted for when considering undertaking a basement extension in London. 

5. Excavation and groundworks 

Typically, the ground works associated with a basement construction will account for between 14-44% of the total cost of your basement. This process will typically last between 12-20 weeks and so as you can imagine this is a significant chunk of the total work that will be carried out. 

Within this 12-20 week period, you can expect your site safety structures to be erected. Then, an initial external opening to the basement area can be created. At this point your existing foundations may need to be underpinned and temporary weather-proofing and supports can be installed where necessary. This is all key in order to ensure the structure is safe. Then, digging can begin, with the excavated ground brought to the surface and disposed of. Any electrics, waterworks and sewage works may also be rerouted at this point if needed. 

Once this part is complete, waterproofing will be needed – but your waterproofing requirements will be specific to your ground type, location, surroundings and more. 

6. The build, finish and additions

Of course, every basement is different – so the cost of this part of the build is dependent on the size, structural requirements, design specification and intended purpose for your new space. For example, you may be installing a kitchen diner, gym or swimming pool. This is the point where the £5,000 per square meter ballpark figure can really be blown out of the water, so if you’re creating something particularly special then be prepared to dig deep! (No pun intended!).

If you’re thinking of starting on a basement build then we highly recommend considering insurance at the very first stage. At Ellis David, we are specialist insurance brokers and can offer both impartial and expert advice to make sure that you have the right insurance in place for your basement extension project. Get in touch with our team today to find out more.

How much does it cost to build a basement in London?

A completed basement can cost anywhere between £3,000-£5,000+ per square meter, but with so many factors that can influence the cost of your extension, from the size you have in mind to the ground conditions, waterproofing requirements and more, this is nothing more than a ballpark figure. london basement extension