Cellar Conversions: The Design Essentials

Converting and extending a cellar can provide valuable extra living space without drastically altering the exterior of your home. Editor in Chief Michael Holmes takes you through some of the design choices you should consider.

Headroom: There is no minimum ceiling height for basement ceilings under the Building Regulations, but a practical minimum height is 2,100mm, and the taller the ceilings the better. If the basement is to form a separate dwelling, then most local authorities will apply a minimum ceiling height when determining the planning application of 2,300-2,400mm.

Structure: If the ground is being excavated or the basement extended, then existing walls are likely to need underpinning and the floor structure supporting using timber or steel beams. This will require design and calculations by a structural engineer. The structure will also have to meet a minimum half-hour fire rating.

Means of Escape in Case of Fire: If the basement is to be used for habitable purposes, an external door or window suitable for egress should be provided with an openable area of not less than 0.33m2, measuring at least 450mm x 450mm. Alternatively, a protected stairway leading from the basement to a final exit can be provided.

Ventilation: The Regulations require ventilation to be designed to take into account moisture content in the structure and air quality in the basement. Mechanical ventilation is the most likely solution unless there are sufficient external doors and windows.

Stairs: Stairs to the basement should have a pitch of not more than 42° and have a headroom of not less than two metres, with suitable handrails and non-climbable balustrading with a gap of less than 100mm between spindles. Location of stairs needs careful consideration for space efficiency.

Thermal Insulation: Insulation should be provided in external walls and floors. The insulation should have a reasonable thickness and be compatible with the tanking material. All doors and windows must comply with Part L with a U-value of 2.0W/m2K or less.

Lighting: Wherever possible, bring natural light into the basement by excavating light wells, using sunken basement windows or sunpipes. Using borrowed light from above, down the stairwell or via glass flooring will also help ensure the space does not feel like it is below ground level. Where natural light is limited, use lots of artificial lighting to avoid any feeling of darkness. Energy-efficient lighting should be fitted where possible.

Waterproofing: Whatever method is used it should be appropriate for resisting ground water pressure and should have British Board of Agreement (BBA) or similar independent technical accreditation. Most basement specialists use a cavity membrane system. This is a textured breathable membrane around the entire basement behind which any damp is trapped and channelled down the outside of the basement walls, under the floor and into a sump from where it is pumped harmlessly away. The alternative is to create an entirely watertight structure using chemical tanking painted on or applied as a render, or as a series of continuously joined waterproof membranes.

Drainage: If the mains drains are below the new basement level and it is possible to achieve a fall of one in 80, then a conventional connection is possible. If the floor level is below the mains drain and you want to include bathroom facilities or a laundry room, you will need to have a pumped drainage system. Small-bore systems combined with a macerator may help to solve drainage issues and increase design options.

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