The Hopps Partnership have been privileged to work with some of the foremost basement contractors in London and over the last decade we have seen the number of basement excavations increase almost exponentially. As such we have found ourselves in an ideal position to advise on the impact a basement may have on a property.
The answer to the question of safety is that the vast majority of basements that are excavated do not end in with any serious damage despite media attention which would suggest otherwise. The Hopps Partnership have worked on many hundreds of basement schemes and whilst we have had to deal with damage, this has been decorative in nature in almost all cases. Evidence suggests that a basement excavation is a safe form of extending your property, as long as it is undertaken by a competent contractor, guided by a robust design team with as much information as possible on the soil type that the subject property sits on.
So what does a basement involve?
When you want to form a basement beneath your property, you will invariably start with architectural plans showing the layout and location of walls, stairs, services etc. The next step is normally to engage an engineer who will design the structure and finally a builder will need to provide design and construction information on how the work is undertaken safely. In many ways, the contractors method of undertaking works is the most important, if this is not carefully thought out then damage can indeed occur.
As with all construction, there are many ways to undertake the work but for the main part digging a basement is the process of forming a concrete box beneath a property. This cannot be done in one or even a small number of stages but rather is normally undertaken in many small operations that constitute what is known as underpinning. Underpinning is the process of forming small sections of concrete wall beneath an existing wall/structure.
The above image shows an underpin in the process of being constructed. To the left and right are completed concrete underpins, above to the top of the image is the original brickwork of the house and below is metal reinforcement which is tied into the adjacent concrete. Within the space with the reinforcement currently sits, concrete will be poured thus finishing this section of wall. It is very important to only undertake small sections of work and provide lots of temporary propping to stop any movement of the adjacent soil and the wall above and this is where it is most important to have a competent contractor on board.
Once a concrete box is complete it is necessary to waterproof it internally. This is normally achieved with a drained cavity membrane, an egg carton shape thick plastic that is wrapped on the walls and floor. Water that gets through the concrete will drain behind this to a pump and then to the sewers. Stud walls are then formed in front of this hiding the concrete and membrane behind your newly decorated walls. Below is an example of the most common type of membrane.
What do I need to do under the Party Wall Act?
In almost all cases a basement will require formal party wall negotiations prior to you starting the works or indeed we can advise you if your neighbour is proposing to undertake a basement excavation.
This is because even if you are not connected to your neighbour physically, the Party Wall etc. Act 1996 requires you to serve an Adjacent Excavation Notice under section 6 of the Act where you are digging within 3m of your neighbour and to a lower level than their foundations.
Basements are understandably complex and so we recommend that you engage a Party Wall Surveyor as early as possible to begin collating the information and liaising with your neighbours.
Feel free to call The Hopps Partnership for some free initial advice, we would be happy to discuss your requirements.