Digging a new basement

If you want to expand your property’s size, even up to 50%, but you’re lacking space, maybe it’s good to think of going… downwards.

Even if it’s generally much cheaper and easier to build a loft extension instead of a new basement and it’s also easier to provide it with natural light and fresh air, sometimes the only way to extend the space of your house is to dig a basement, as for certain reasons you may not be able to extend your property upwards.

Digging a basement under your house can also be a good idea if you want to raise your property’s value. When living in London, it is cheaper to dig a basement than to move to a bigger house. The number of applications for house basement in London is gradually rising. However, before actually digging down, house owner has to take into consideration many aspects, which include finding out whether their property is suitable for this type of extension.

For this, you need to have a site investigation carried out. Some of the properties are unsuitable for a basement extension primarily because of their location.

After a site investigation has been carried out, you may come across several problems.

High water table is an issue that causes the biggest risk when creating a new basement. By installing drainage system, you can lower the water table, but it can possibly have negative effect on other buildings in the area. The high water table is related to waterproofing, which we will cover later.

Other conditions you should take into account when digging a basement are: soil type, presence of natural gases, evidence of ground contaminants, movement risk, boundary conditions, existing structures and services, and sometimes even archaeological finds.

Soil type is really important when it comes to waterproofing, because certain types are easier or harder to dry. For example, clay-based soil would drain much worse than chalky soil. Also, in urban areas, soil may be contaminated with hazardous materials, such as lead or asbestos.

The type of waterproofing must be chosen carefully for your basement’s needs, depending on water table and the soil type. Unless you want to create a cellar under your house, your new basement must be properly waterproofed. There are three types of waterproofing protection you can install in your basement – barrier protection (type A), structurally integral protection (type B) and drained protection (type C).


Type A waterproofing protection is also referred to as tanking. It provides protection from ground waters by applying a waterproof material to internal or external walls.


Type B is when protection is provided by the material itself which in this case it is cement mixed with waterproofing additives.


Type C is basically a drainage system, which is the most common waterproofing protection.


Usually there are two of these types combined being applied to a new basement, which is insisted on by independent insurers that are insuring the new build. After choosing the right waterproofing system, you have to make sure it is done correctly.

Basement needs to be also ventilated properly, to comply with Building Regulations. It’d be the best if your new living space had access to fresh air directly from the outside through windows – which would be the source of natural daylight as well. It is not always possible, so when you can’t install windows, you’d have to invest in mechanical ventilation system that would blow the air into the basement. There are also setups that switch on automatically whenever the humidity level goes too high.

When digging a new basement, you also have to think about providing it with some sort of lighting – again, the best would be to do it by digging up a light well and installing windows, which is the most beneficial option. Sometimes however it is not possible. You can always try using sun tunnels in your basement. If having any access to sunlight is impossible in your case, the best thing to do is to just use bulbs giving light similar to natural one.

Before actually digging a basement, you must  be prepared to get in touch with an architect and engineer, gain building regulation approval for the excavation, comply with the fire regulations etc. Creating a new basement is a process that needs time for preparation and cannot be done without going through many phases including some paperwork.

The current basement policies can be found on your council’s internet site along with a guide on how to submit a planning application.