Basement Conversion : How To Convert A Basement/Cellar

Whether you want a new bedroom, office, playroom or even a home cinema or gym, converting your basement is a great way to add space to your home.

basement conversion


The lack of space and the high cost of building, make it really difficult in the UK to build big houses. But this could easily be solved by adding another storey to every new home by converting a basement or cellar.

Basement conversions are often less intrusive alternatives to loft conversions or extensions. Gary White, who works for basement conversion company Storm in Bristol, points out that: “Basement conversions have become more popular recently. If the basement is already formed then it tends to be a lot cheaper than converting a loft or adding an extension.”

Not only is having a basement conversion a good alternative to moving, it can increase the value of your home. Bobby, a basement conversion specialist, says that the amount it costs to convert a cellar is normally equal to the value it can add to your home; “Depending on where you live and the types of houses in that area,” he adds.

What can I use a basement conversion for?

Basement conversions can be used to create almost any kind of space: a utility room, playroom, kitchen, office, or even a pool, gym or spa. If you already have a basement then converting is a likely option. If you don’t, one can be dug out but the process is painstaking and very expensive.

Planning your basement conversion

There are different stages for planning a basement conversion. The first step is to think about how you want to use the new basement. This will determine the need for installations such as water, electricity or subfloor heating.

The next step is to consider the individual basement conversion cost, which will depend on the existing structure in your basement, the number of walls that will have to be taken down or be newly built and on the individual installation and flooring ideas.

An architect or a building contractor can estimate the individual costs for structural alterations to your basement, as well as costs for insulation. For further installations you can consult a plumber or an electrician for a cost estimate depending on your ideas.

Benefits of a basement or cellar conversion

Converting a basement is a great way to create additional space without losing any of your garden. The other positive aspect  of converting a basement is that it won`t regularly alter the building`s appearance or if it does the change will not be very noticeable.

Looking to convert your basement into a more vital space. Follow the link bellow!

Moisture prevention

Many of the affecting factors are related to the prevention of moisture. It is so important issue, that here is some basic information about it for you. You will have two options to choice for avoiding the dump and moisture in the structure of the basement: waterproofing below ground level aka tanking and cavity membranes.

basement waterproofing

Waterproofing your basement

This damp proofing solution works by leaving a gap between the wall and a waterproof membrane, which allows any excess water to drain down the gap and be pumped out. Put simply, Bobby says: “You can think of the basement like a fish tank – it’s sealed so no water can get out but also none can get in.”

The waterproofing will be made usually by cementitious waterproof render system on the walls, typically applied in several layers, linked to a waterproof screed on the floor. It can also involve a sheet membrane, asphalt or other liquid-applied waterproofing materials.

Tanking is also required to withstand the external water pressure around the cellar. The water table causes pressure around a basement and it can be so enormous that unless the tanking is very securely fixed to the substrate, it can fail.

Several studies consider cavity membranes to be the most reliable way to waterproof a basement. The membranes are used to create an inner waterproof structure in the basement or cellar, behind which is a cavity that is fully drained. Thanks for this structure any tiny leaks in the outer structure are diverted harmlessly away via a drain.

Other things to consider

There are many other remarkable factors, which can increase the cost such as the need to move services, e.g. relocating electricity or gas meter, and also if you are adding lightwells and a new entry, and lowering the floor level to increase ceiling height. As you can guess, all these phases will be quite expensive. So high-quality planning is really essential for your basement conversion project.

Basement lighting – Lighting your basement conversion is key to making sure that the space is appealing. Good use of spot lights throughout the room is one way of achieving this but you can make use of natural light as well. Light wells allow sunlight to flow through into the basement or you could dig down into your garden to put in windows.

Heating and ventilation –  Heating and ventilating a basement is relatively simple, as radiators can be put in and linked up to your main central heating system easily and you can use extractor fans to ventilate.

Plumbing – Plumbing can be put in to allow for an extra bathroom in a basement conversion. Even if there is no existing drainage there, a pump system can be used to get rid of waste.

Do I need planning permission for a basement conversion?

Basement conversions without planning permission

Most simple basement conversions do not require planning permission. This includes waterproofing installation and the installation of electricity and plumbing. Therefore, ifyou do not plan on any alterations to the existing structure, planning permission for basement conversion will not be required. Lowering the floor of the basement most likely won’t require planning permission either, while digging a new basement most probably will. The building constructor or architect you hire for your basement conversion will be able to tell you whetherpermission is necessary for your plans or not.

Basement conversions that requires planning permission

Even though most basement conversions can be done without planning permission, in some cases planning permission for basement conversions is required. This includes basement conversions that would alter the external appearance of the house or change the use of the basement, as well as rather extensive work such as digging a new basement under an existing house or changing the original layout of the basement. For listed buildings and buildings in Conservation Areas planning permission is generally required. Information on whether you need to apply for planning permission and on application details can be provided by your local Council Office.

Other regulations for basement conversions

Even if you don’t need planning permission, a number of building regulations may apply for your basement conversion. Basement conversions that include the creation of new bed or bathrooms or other rooms except storage rooms will be required to meet a number of building regulations regarding fire safety, ventilation, electricity and plumbing. Also, a number of regulations regarding structural demands may apply.

Cost versus value

A basement conversion can give you a 69% return of investment according to Houselogic, as it adds valuable living space to your property without increasing the footprint of your home.

However, estimating the return of investment of a basement conversion is quite complicated as it always depends on the local property values and the total cost of conversion. The basic rule is, that it almost always makes financial sense to add lower ground floor space in high-value areas, such as central London and other sought-after urban centres.

How much will a basement conversion cost?

The average cost of a basement conversion is rather higher comparing to other extension projects. It is also a very challenging home improvement projects that require skills and proper planning.

An idea of the costs involved:

Conversion project Cost per square metre
Convert an existing cellar £750 – £1,400
Lower the floor level and underpin an existing basement £1,500 – £2,000
Digg and underpin a new basement space £2,000 – £3,000
Digg a new basement space underneath a garden £1,500 – £2,000
Create a lightwell / external access: £5,000 – £7,500
Engineer’s fees £1,000 – £1,500
Planning permission(if required) £170

Diverting drains

If you have to divert the drains and conditions are difficult such as solid rock, clay, sand, peat or marsh or if the local water table is high affecting the need the constant pumping during construction, which are the factors, which will increase the costs of the basement conversion. And it is good to notice, that access for removing the subsoil is another important consideration that will affect costs.

Factors affecting cost

As I mentioned above there will be several factors, which affect the costs of the basement conversion. Here is a summary of those factors:

  • The summary of the affecting cost factors
  • The need to divert drains beneath your house
  • Your home has solid concrete rather than timber subfloor
  • The difficult ground conditions such as clay, made-up ground, sand or marsh
  • The local water table is high => necessitating constant pumping
  • Access to the site is poor

How to find a builder?

A basement conversion is a demanding project. A very basic conversion such as converting a single-room cellar using a membrane lining system can take just two or three weeks, including digging out the sump and fitting the pumping system. It sounds pretty fast, but requires a high-quality knowledge of basement conversion as the bigger projects. This is the edge point of the whole project and when you have hired a specialist for your project you can be sure, that the project will go forward as planned.

You need to choose a specialist basement contractor with experience of waterproofing system you have chosen. Search the website of The Basement Information Center ( for a company registered with the British Structural Waterproofing Association. Look for an insurance-backed guarantee that will pay for repair works even if the contractor goes out of business. You can get more information by calling 01276 33155.

Other regular instructions for you to help to find a proper contractor for your project are as follows:

When you are starting to plan your basement conversion project and you are not sure whether you need to apply for permission for your basement conversion, consult your local administration, a basement conversion company or architect. They will help you in that case. But they will be right contact to find a proper contractor for your project, too.

Here is a memory list for you, how to find a contractor to your project:

  • Stay local choosing the company.
  • Ask your insurance company for recommendations.
  • Check credentials.
  • Check the contractor`s license and insurance.
  • Get it in writing.
  • Don`t pay for labour up front.
  • Ask for copies of receipts.