A Complete Guide to Basements

With living space at a premium, and the cost of relocating growing ever more expensive, especially in city areas, people are looking for new and innovative ways to extend or expand their homes. Curiously enough, for many, one way of expanding a property has literally been right under their noses all the time, and that is to enlarge an existing basement, and if the property doesn’t have one then there are specialist companies that will put one in.

Older London properties in particular were often built with basements and these had several uses, such as storage for household goods or for coal, and more elaborate basements might have been used as the kitchens for a more affluent family. The basements often suffered from extensive damp and flooding problems and could be cold and unpleasant places to live or work.

A Basement under construction

A Basement under construction

Converting a basement

Planning & building regulations

Converting an existing residential basement in living accommodation is unlikely to need planning permission as long as it not separate from the main house and there is not a significant change of use. If a light well is added which changes the external appearance of the house it may require planning permission. Any building works to a property will automatically require building regulations approval and for this type of work it is advisable to seek professional advice before commencing as there are important building regulation issues to deal with, such as underpinning of the foundations, fire escape routes, ceiling heights and ventilation to name just a few. If the property is not detached then there may also be party wall issues to deal with before going ahead with the work.


The first and biggest threat to such a project is water. Naturally any area that is below ground level is subject to the laws of gravity and water is inclined to make its way to the lowest level it can find. Consideration also needs to be given to the water table in relation to a basement as a higher water table will have a greater impact on the ability to waterproof the rooms. Another issue to face is the possible lack of direct sunlight, making it more difficult for any damp to escape or to be dried by the warmth of the sun. These conditions are ideal for the development of mildew or other damp related problems. But converting a basement to living accommodation today has been made a far more attractive proposition with modern building practices.

A builder waterproofing a basement

A builder waterproofing a basement

Outside Topography

The first area to look at when planning a basement conversion is the outside topography surrounding the property. A thorough check of all the downpipes and gutters on the house needs to be done to ensure there are no leaks or leaf jams as these may cause water to pool around the property which could seep into the basement area. These would also need to be checked on a regular basis.

Rainwater may also seep from the outside boundaries of the property toward the basement and the ideal way of dealing with this would be to slope the house boundaries away from the house and toward the garden. If this is not possible or the cost is too great then another solution would be to place a grid drainage system or a French drain around the perimeter of the house which would carry any water away from the basement. A harder problem to rectify is water seepage from ground water and the only solution may be to install a pump pit in the basement which would carry the water away thus ensuring a waterproof basement. By approaching the potential problems from the outside of the property first it is possible to avoid the potential of just diverting the problem to another part of the house.

This is a basement in the process of being waterproofed

This is a basement in the process of being waterproofed


Turning our attention to the inside of the basement may find a damp room or rooms that have not been in use for many years and these will need to be waterproofed to allow them to be used as living areas and will most likely need to be dried out and ventilated. Simply coating the inside walls with water proofer would not be enough to guarantee a long term water free basement. Waterproofing the inside of a basement may involve several techniques combined to create a habitable space as each project is unique.

Also involved may be the technique known as tanking which can take the form of cementitious materials applied to the walls in the form of slurry which when cured controls the ingress of water through the walls. Other methods are designed to create a cavity membrane by means of a mesh or fibre glass or another method to achieve waterproofing is by installing a high density plastic membrane with an air gap behind which allows for the free movement of water that can be channelled into a drainage system. This is then plaster boarded and finished with plaster.

It may better to use a professional company to carry out this work as they will be able to assess the needs of job and will also provide important certification for the work.

This is what can happen when water gets into your basement

This is what can happen when water gets into your basement

Building a new basement

Because of the high cost of building plots and plot sizes becoming ever smaller, the UK apparently builds the smallest homes in Europe, and it has been estimated that less than two percent of homes being built today in the UK have basements designed into the plans. But that figure is set to change as single building plots become harder to find and also more and more people are making use of sloping sites which afford the opportunity to build a partially submerged basement. An increase in self builders has also contributed to houses being individually designed with more thought going into maximising the space available. With new planning directives from central government and the drive to increase housing densities we may well see more volume house builder’s move into including basements in their design portfolios.

Adding a basement can also be a solution to a site that has height restrictions built into the planning permission. That can mean that a bungalow has the potential to become a two storey property. It has also been said that a basement can be ten percent more thermally efficient than the rooms built entirely above ground. This can also mean savings in the quantity of insulation materials used.

Besides the opportunity of designing a basement into the plans for a new build there are many people who are realising that they can extend their home by adding a completely new floor below ground. This is a job for specialist companies as it involves undermining the existing foundations of the property. Many will offer a design and build service as the planning and design work can be more involved than building a new property, and modern construction methods are making the option a pleasant possibility.

Building a new basement in an existing property will involve the removal of a great deal of soil and some serious underpinning, and it is most likely that you will need to move out of the property for a time whilst the major works are being carried out. Modern building materials used for the structure can be used to play a large part in making the basement waterproof, but the techniques for waterproofing will be much the same as those for converting an existing basement. The same considerations will also need to be given to the exterior of the property to ensure that waterproofing will be effective.

Think of any room in the home and those that you don’t have but would like to, and there you have the potential for a new basement. You may wish to have just one room or depending on the area available it may be possible to have a self contained property offering several bedrooms along with a kitchen and bathroom, or perhaps you are looking to improve your home by adding a pool along with a sauna room. Whatever your ideas may be, a basement has the potential to fulfil your dreams and at the same time pay you back in dividends for the increase in value. Adding a new basement to a terraced London property may cost as much as £500,000 but could increase the value of the property by as much as £1,000,000 depending on the location.

Kitchens and Bathrooms at Lower Levels

Adding kitchens or bathrooms to a level that is lower than the existing plumbing and drainage is not a problem as modern pumping systems will allow the waste to be carried away with little effort.


There are many websites advocating the benefits of basements while at the same time promoting the potential of carrying out the work as a DIY project. Whilst it is possible to be greatly involved in a basement conversion it is one of the most complicated projects to have done and it is essential at the very least to have professional advice from structural engineers and waterproofing specialists. The costs to remedy problems that arise from failed waterproofing work can be scary so make sure that you do your homework.