London Basement Conversions – What can go wrong?

We’ve talked endlessly about London basement conversions… that’s only because we love converting basements, our customers value our experience and knowledge about basement building…and if we say so ourselves, we are very good at it. The basements we have built have ultimately put a smile on our customer’s faces; it’s that space they have envisioned that adds the ‘wow’ to their home, whether it’s a space of calm or a space to entertain. That doesn’t mean however, that the process of building these basements have been completely smooth-sailing. We have experienced several challenges in some of the conversions we’ve built. The key, of course, is to overcome these challenges by finding workable solutions.

If you are thinking about converting your basement, be aware of some of these potential roadblocks along the way, but do remember there is always a solution to every problem.

  1. Planning permission

Excavating a basement is a major structural undertaking and will require planning permission via your local council. Make sure you appoint an experienced architect who understands your local council’s requirements. Plans that are not detailed and do not abide by council regulations are likely to get rejected. If you are in a conservation area, you will need to take additional rules and conditions into considerations. In most councils, the consultation is 8 weeks from the time of submission of the plans. Any changes will cause the 8-week period to re-start. It’s important to get it right the first time.

Having said that, in Kensington and Chelsea there have been over 800 planning applications for basements submitted in the last 5 years. Only 10% are rejected and these are almost always resubmitted. So, the odds are in your favour.

  1. Love thy neighbour

You will probably have to enter into a party wall agreement with your neighbour. We suggest you involve your neighbour in your plans to get them on side before you put your planning application in. If they object after the plans are in, you might have to amend your plans and restart the 8-week consultation period, further delaying your project.

  1. Risk of groundwater

The impact of groundwater driven by both the general water table and often hidden underground streams can throw a spanner in your works. Encountering water adds to the complexity as excavating wet clay takes longer, can clog up conveyor belts and requires continuous pumping.

  1. Diggability

It’s also more difficult to install basements where there is a solid floor. There are some cases where it really is not feasible to fit a basement. In fact, many modern terraces and townhouses that are built on raft foundations can’t be underpinned, so can’t have basements. It’s also important to have good access for the excavation work: too close to a boundary or the road and it could be difficult for a JCB to get in.

  1. Waterproofing or tanking

We believe this is one of the most important aspects of basement building. Poor waterproofing techniques resulting in leaks and floods can destroy all the hard work in converting a basement into a beautiful living space. Problems associated with this are:

  • Failure to properly seal service penetrations in basement walls.
  • Poor backfilling which can lead to punctures in tanking.

One of the solutions to this problem is to install a cavity membrane system.

SDA Build London is proudly accredited by the UK’s leading waterproofing brand, Delta, with all our basement conversion projects installing their cutting-edge Delta Membrane Systems. This high-density polyethylene covering is applied to the walls, floors and ceilings of a basement dig out, where it blocks, controls and drains any potential ground water ingress from entering a property. It can be applied to new, existing and retrofit projects, and is flexible and durable so it can cope with movement or vibration in the home – important as this is often the cause of basement flooding! All our Delta Membrane installations come with a 30-year warranty that ensures your basement is protected.

If you are considering building a basement, give us a call. We’ll be happy to visit your property, discuss your ideas and share our thoughts on the feasibility of a basement on your property, any potential pitfalls and how we can overcome them.

Call us on 0208 191 7595, email us at or book a Free Refurbishment Consultation via our website. We’d love to hear from you.