basement conversion

Everything You Need To Know About Basement Conversions

20 September 2021

We often get asked a whole load of questions regarding basement waterproofing, so we thought it was a good idea to put together a blog post all about basement waterproofing, answering all of the most commonly asked questions! If you have a cellar area under your property then the potential to carry out a basement conversion is there.

What is a Basement Conversion?

A basement conversion is the converting of an existing below ground area which involves only the change of use and therefore doesn’t require planning permission. However, reducing the floor level of a cellar or creating a new basement would enlarge the volume of your home and would be therefore treated as an extension which would require planning permission.

What Are the Benefits of Converting a basement?

basement conversion

Unlike loft conversions at the top of the house where the potential for creating extra space is often limited, a basement or cellar conversion is far more flexible allowing homeowners to create a range of exciting opportunities.

Everybody has their own idea for a basement conversion; some choose to create an additional room for their property, others are more ambitious choosing to create a luxurious space such as a cinema room or gymnasium.

If you are selling your property, more often than not the potential of creating such high-quality spaces can also speed up the selling process and significantly increase your properties value.

Whether or not converting a basement is financially feasible will be dependent on the cost of the conversion comparative to the property value.

In areas where property value is high, such as central London, it is financially a good idea to extend your property underground, increasing the property value further.

If considering a basement conversion in a lower-priced area, careful consideration and analysis needs to be carried out to ensure the increase in property value will outbalance the cost of the conversion itself.

You should also consider if the desire for a basement conversion is out of practicality or to increase the value of your home. It may even be for both of those reasons.

Most estate agents confirm that a basement conversion will add between 10% and 15% to the value of an average household. (Source: Savills) The value per square metre rate of a basement can also be affected by the area in which you live as well as factors such as access and the potential for natural light.

Additionally, from a rental perspective, converting your basement into an extra bedroom is a great way to increase passive income whilst adding property value.

Planning Permissions for Basement Conversions

The good news here is that unlike outwards property extensions, most basement conversions don’t need planning permission because you are only changing the inside of your property and not altering the external appearance in anyway. However, if you wish to add windows or a new entrance to the property, planning permission may be required.

What Are the Building Regulations for Converting a Cellar?

Any conversions to a cellar with intent for it to be used as a living area is covered by Building Regulations. The purpose of these regulations is to ensure that buildings are safe, hygienic and energy efficient. The regulations for a basement extension include ventilation, sufficient fire escape methods, wiring, ceiling height and arguably the most important, waterproofing.

Renovating an existing habitable basement, or the repair of a cellar that does not involve a change of use is excluded from Building Regulations.

Party Walls Agreement for Converting Your Basement

A party walls agreement is a legal framework that resolves and prevents any disagreements that are related to the party walls that are next to a neighbouring property. This act was introduced in 1996 and you will need to speak to the owner of the adjoining building before the conversion of the basement can begin.

Waterproofing a Basement

Typically, basements and cellars are often left cold, damp and unfurnished and not up to the same standard of finish as the rest of the house.

To convert a damp basement into a warm habitable area is a job for a waterproofing specialist armed with a high level of waterproofing knowledge and know how. A successful conversion is accomplished by using a variety of basement waterproofing techniques that we at Timberwise specialise in.

As basements are below the water table there is usually a protective coating on the exterior walls below ground, but, over time, this coating often deteriorates allowing water to penetrate through the walls, making the basement susceptible to damp or flooding.

To remedy such problems systems are put in place to control the direction the water takes when it enters the house and to keep it separate from useable areas.

To understand the various ways that basement waterproofing is carried out then it is necessary to understand the different types of waterproofing systems that are commonly used. There are three main forms of basement waterproofing and they can be used in conjunction with each other to provide efficient protection against damp.


Basement tanking, also known as Type A waterproofing, involves the application of a waterproof cement system onto the walls and floors of the basement. The tanking slurry then bonds with the masonry substrate which provides a protective barrier against water penetrating its way through the building. Unlike using Type C waterproofing which controls the water to flow the property, tanking stops any water from finding its way through.

Cavity Drain Membranes

Cavity drain membrane systems involve the use of a membrane that is applied to the walls, floors and sometimes ceilings of the basement. The cavity between the membrane and the wet wall allows for any water to travel down it towards the drainage channel.

Once the water has entered the drainage channel it will then be collected and channelled into the sump and pump. The water is then pumped out of the sump and pump into the drains. The sump and pump system is vitally important to the cavity drainage system so having it regularly maintained is important to make sure that it lessens the chance of failing.

Having a battery backup system is something we would recommend having due to the fact that if there was a power cut then the system will continue to work. Regular sump and pump maintenance ensures that your newly converted basement is at less risk of potentially flooding.

Do You Need a Specialist to Carry Out Basement Waterproofing?

We would strongly recommend that you have a fully qualified company to carry out all basement waterproofing works. At Timberwise we have fully qualified surveyors and technicians who can provide the best solution for your property as well as complying with the latest BS regulations (BS8102) that apply to the whole of Great Britain.

We also provide long-term guarantees for all works carried out so you can be sure that your property is in the hands of the professionals.

Our team of professionals at Timberwise always ensure that the work is carried out to the highest standard, leaving you with a wonderful new space, free of damp or potential for future damage from dampness.

Ventilating a Basement

It is important that the basement is properly ventilated to remove any stale and musty air so that the fresh air can flow throughout the basement. Opening windows combined with using extractor fans and vents are the most efficient ways of reducing humidity and therefore stopping the build up of condensation.

Heating the Basement

No one likes a cold basement so heating your basement is important to make you and your family as comfortable as possible. There are a number of methods that can be used to heat up a basement and these are:

  • Radiators- Fitting a radiator in the basement is one of the most obvious choices to increasing the heat in the basement. It is important to make sure the temperature of the radiators is regulated.
  • Underfloor heating- Unless you have not converted your basement before, having underfloor heating is one of the most popular methods in basement conversions. One of the main reasons for this being a popular method is due to the fact that it can spread the heat evenly.
  • Electric heaters- Having a plug in heater is not of the most efficient methods of heating the basement but if don’t have any heating at all in your basement then electric heaters are a good short term solution.

Lighting up the Basement

Basements tend to be dark with little or any natural light. Before the basement conversion process has started then this is definitely something you will need to think about. There are a number of lighting options that you can consider that will improve the whole ambience of the basement.

It is unlikely that you will have a high basement ceiling so having hanging lights is not something you should really consider. An ideal solution for basement lighting is introducing spotlights, these don’t take up too much space but provide sufficient lighting. Another solution would be to have wall lighting as well.

Using lamps also helps makes the basement a cosy atmosphere to live in. Combining both ceiling lights and lamps is the most ideal solution to improving the lighting in the basement and the next best option to having natural light.

Having natural light is what you ideally want to have in your basement, it not only makes the basement more welcoming, but it is healthier for you and whoever is using the room. If you are lucky enough to have windows in the basement, then we would recommend making the most of them. An example of this would be if the window has being blocked for some reason. Anything blocking the window should be removed to make the most of the natural light where possible.

If you do not have a window within the basement then fitting one as part of the conversion is certainly an option to consider, although it might require planning permission. You will not need planning permission if you are reinstalling a window that has already been permitted.

How Much Does a Basement Conversion Cost?

As you can tell there are many things you need to consider when converting a basement, all of which increases the budget you will need for the conversion. It is hard to specify an accurate cost of basement conversions due to a variety of factors. These factors include:

  • The size of the basement.
  • The type of basement you are looking for e.g. converting it for living purposes, cinema room, gym etc.
  • Whether you are looking to dig a new basement space under the house or use an existing cellar.
  • Whether the drains need to be diverted underneath the house.
  • Whether the basement is vulnerable to flooding. If it is then water will need to be constantly pumped out.
  • The type of lighting that is to be installed.
  • The type of waterproofing system that is to be used.
  • The type of flooring that will be fitted

This is a long list of costs to consider when deciding on how much to spend on a basement conversion. We would recommend having a qualified professional carry out the basement conversion, unless you are a fully qualified contractor yourself.

How Long Does a Basement Conversion Take?

A common question we are asked by our customers is “how long does it take to convert a basement?” Well, in short, it is impossible to give a definitive answer as each basement conversion has too many unique variables.

If you are wanting to just convert a single room cellar, then this could take only a few weeks – this would include the installation of a cavity drainage system with a sump and pump system as well. However, if you looking to extend the cellar then this could take several months to carry out.

Contact Timberwise for All Your Basement Waterproofing Needs

We are a nationwide company with 50 years of experience in property preservation. At Timberwise we understand that your property is likely to be the largest single investment you will have.

The importance of trusting people with your home is never to be overlooked, that’s why we pride ourselves on developing strong client relations.

This is achieved through going that bit further for our clients; we understand that quality is a standard, not an extra. Exceptional quality is evident at every stage in our clients’ experience with us. We are continually striving to be the best in all that we do, and anything less is not good enough.

For more information about waterproofing your basement then contact Timberwise on 0800 288 8660 or you can request a survey online. A survey through Timberwise will be able to identify the full extent of the problem and determine which basement waterproofing system is right for you.