Cellar conversions

If you are fortunate enough to have a cellar beneath your house and you need more space you may wish to consider converting your cellar. It is important that your conversion project is carefully planned. This guide is not a substitute for professional advice but has been written to provide you with useful information about how the building regulations will affect your conversion.


If you have a cellar beneath your house it may well be suitable for converting. When you are thinking about whether your cellar is suitable for conversion you might like to consider:

  • Is there sufficient headroom in the cellar, bearing in mind that the ceiling and the floor treatments that you will have to install are likely to reduce the available headroom?
  • Does the cellar ever flood?
  • What is the access like? Is there a place for a staircase?
  • Will you have enough storage area if you convert your cellar?
  • Is there enough room in your cellar to provide the accommodation that you require or would you be better extending the property?
  • Is there any ventilation to your cellar or could any be provided?

If you can resolve all of these issues then your cellar may well be suitable for conversion.

Technical issues - how to proceed 

Cellar conversions can be complex projects and unless you are experienced in construction you will need to get some professional advice. 

Lowering the cellar floor

If there is not sufficient headroom in your cellar it is sometimes possible to lower the cellar floor. This is not however a simple operation and careful consideration needs to be given to whether lowering the floor will undermine the house or the neighbour’s house foundations and whether the floor will end up below the water table and make the property more vulnerable to flooding. Specialist advice should always be taken before considering lowering cellar floors.


As most cellars are set within the ground they tend to suffer from problems with damp. If it is to be converted to a habitable room your cellar will need to be damp proofed. A number of systems are available for damp proofing cellars and most of them use a proprietary waterproof render system known as tanking. This is applied so that it forms a continuous damp proof layer across the floor and up the walls and is generally installed by specialist companies who

will offer an insurance backed guarantee for the installation. Protecting your cellar from damp is an important part of the cellar conversion process and we will need to approve details of the system that you are using and the installer prior to installation.


Some cellars already have good stepped access to them, whether the existing stairs will provide suitable access to a habitable room is a matter of judgment and our Building Control Surveyors will be happy to offer advice. If there are no steps, or if the existing steps are inadequate, a new stair will need to be installed. Careful consideration should be given to the best location for the stair and this will be influenced by a number of factors including the layout of the existing house and cellar, the headroom available and whether a secondary means of escape can be provided from the cellar area. Wherever the stair is installed it should be designed in accordance with the following guidance.

Pitch: the maximum pitch for the stair should not exceed 42 degrees.

Rise and going: the maximum rise of each tread of a domestic stair should not exceed 220mm and the going should be at least 220mm.

Headroom: the clear headroom over the stair should be at least 2m, for some loft conversions the building regulations allow a reduced headroom of 1.9m over the centre of the stair and 1.8m on the outside edge.

Handrails and balustrades: the stair should be provided with a handrail at least 900mm high and any exposed edges of stairs or landings should be provided with balustrading at least 900mm high.

Fire precautions

Your cellar will need to be provided with suitable escape routes in case of a fire. If your cellar has a light well it may be possible to upgrade this so that as well as providing ventilation to the room it can provide a secondary fire escape. To be considered as a secondary fire escape it would need to be fitted with a door or window with a clear area of at least 0.33m2 and 450mm wide. Special fire escape hinges should be fitted to this window to ensure that it can be fully opened if you ever need it and you should be able to easily climb up from the light well to ground level. If you cannot provide a secondary fire escape the staircase will need to end up in a fire protected hallway with a door direct to outside. When converting your cellar it is a good opportunity to review the fire precautions that are available in the existing house. Mains operated smoke detection significantly improves fire safety in the home and the building regulations require that it should be installed where cellars are converted to habitable rooms.


If you are looking to include a sink, bathroom, shower room or even a washing machine in your conversion it is important that you consider drainage at an early stage. Any new appliances will need to connect to your existing foul drainage system and the drains are usually above the level of the appliances that you wish to install. You are likely to need to install a pumped drainage system and various package systems are available that macerate the drainage and pump it via a small diameter pipe to the existing drainage system. When planning your layouts make sure that there are suitable routes for pipes to run to a point where they can connect to existing drains.

The ceiling

Unless your existing cellar has an adequate ceiling you will need to provide one as part of your conversion. Plasterboard is the most common material used for ceilings as it offers good fire resistance and flame spread properties. Other materials can be used but they will generally need to be treated to improve their fire performance. If either your cellar or the room above is to be used as a bedroom you will need to install 100mm of acoustic quilt within the floor void between the rooms.


To maximise the usability of the room you will probably want to install heating, in most instances the most effective way of doing this is to extend the existing central heating system. You will need to check with your plumber or heating engineer to ensure that your existing boiler and pump has sufficient capacity to serve any additional radiators. Any new radiators should be fitted with thermostatic valves to control the room temperature. If it is not possible to extend the existing system, or, if you prefer an alternative method of heating, e.g. electric panel heaters, careful consideration should be given as to how these can be switched and controlled to ensure that they function efficiently.


Any new habitable rooms will need to be ventilated and this can sometimes present a problem for cellar conversions. Where the cellar contains a light well this can sometimes be adapted to include an opening window equivalent to 1/20th of the floor area of the room with a trickle vent at high level. In any bath or shower rooms an extract fan should be fitted and if they do not have opening windows extract fans should be fitted that are triggered by the light switch with overrun timers that allow the fan to remain on after the light is turned out. Where natural ventilation through windows is not practical a mechanical ventilation system will need to be installed and various package systems are available on the market. Ventilation should be considered early in the design stage as, if mechanical ventilation is required, the duct work will need to be accommodated.

Thermal insulation

CO2 emissions are a major concern in today’s environment and you will need to provide a high level of insulation within your conversion. Your cellar should provide an insulated envelope so that the amount of heat escaping is minimised. The walls and floor of your cellar are generally lined with insulating boards and the windows should include 24mm double glazing units incorporating low emissivity glass. As well as insulating your extension you will need to consider the efficiency of any services you put into it. Low energy light fittings should be used where possible and any new heating systems should work to high levels of efficiency and have suitable thermostats and controls.


You are likely to require some electrical alterations as part of your conversion. Depending on the age and condition of your existing electrical system it is sometimes possible to extend existing circuits but sometimes new circuits and even a new distribution board will be required. It is a good idea to get advice from a competent electrician at an early stage. When appointing an electrician please ensure that they are able to issue you with BS7671 test certificates when they have completed their installation as these will be required before your building regulations Completion Certificate can be issued and you will incur additional costs if the test certificates have not been provided.