Building Control guidance - basement conversions

Basement conversions require a Building Regulation application (i.e. a Full Plans Application or Building Notice submission), where the intention is to provide:

  • An extra bedroom
  • A bathroom or en-suite
  • A playroom
  • A study/office
  • A fixed staircase

Why inform the Building Control Service?

Building Control will help achieve the construction standards set out in the Building Regulations. Future sales or properties where work has been completed without Building Regulation approval can be difficult.

It is generally considered that basement conversions are the most expensive of domestic building work. They should be designed and constructed by people aware of the health and safety aspects affecting building occupiers. The following points must be considered and catered for in basement conversions.


Proprietary tanking systems can be used and should have BBA or similar independent accreditation. Cementitious render can be used if the system has BBA or similar accreditation. Liquid applied waterproofing material can also be used and in this case an adequate loading construction must be used to reduce the risk of it being blown off the walls. Any other system that is proposed should be backed by a suitable third party insurance guarantee.

Means of escape in case of fire

The conversion of a cellar into a habitable room does add to the risk to the means of escape in case of fire in a dwelling.

The ceiling areas within the basement may need to be upgraded to be fire resistant.

In some circumstances a basement should be separated from the rest of the house using an FD20 fire door.

In some circumstances, (particularly where the basement is accessed from a room at ground level) an escape window or a door giving access directly to outside should be provided. A person escaping from a window or door from the basement should be able to get easily to ground level outside of the house and move away from the house. Suitable steps with a recommended pitch of 42 degrees should be provided from the window or door well.

An escape window is one that has a minimum area 0.33m² with no dimensions less than 450mm, the bottom of which is situated not more than 1100mm above the floor.

Smoke alarms

Mains operated and interlinked smoke detectors should be provided to the ground floor hallway and first floor landing. If the basement becomes a kitchen a heat detector should be provided to the kitchen.

Ventilation to the room

Rapid ventilation (an opening which is not less that 1/20th of the floor area) and secure and controllable background ventilation (8000mm² for habitable rooms and 4000mm² for kitchens, bathrooms and utility rooms) should be provided. This is the usual standard for new rooms.


New stairs to the room should be as the standard for new stairs i.e. 42 degrees pitch with ideally 2.0m headroom, with suitable handrails and non-climbable and with openings that a 100mm sphere cannot pass through.

Guarding to the new or enlarged window/door well

Where the difference in levels is 600mm guarding should be provided to prevent people falling into the well. It should be 1100mm high, non-climbable and with openings that a 100mm sphere cannot pass through.

Thermal insulation

A reasonable thickness of insulation should be provided to the existing walls and floor, meeting current new build standards where practicable. The insulating material must be compatible with the tanking material. New windows and doors should be to today's standard i.e. a "U" value of 1.6 Wm²/k. The new room should be fitted with a light that is energy efficient.