The space beneath your existing house may be the ideal area, enabling you to create new rooms and much needed living space.
Consideration should be given to the use of the structure and achieving the appropriate environmental grade as detailed in BS 8102: 2009. Heating, ventilation and air conditioning may be required and should be designed at an early stage.
Depending on the size of the property, the amount of basement excavation and building work required and its difficulty, the work for constructing a basement conversion will last typically for 12-20 weeks.
The basement excavation work will also normally require the services of a Party Wall surveyor at an early stage if the basement is to be excavated beneath the level of the existing foundation. The works must be inspected by Building Control and your basement design will need to comply with Building Regulations with regards to means of escape and fire safety. Further upgrades to other parts of the property may be needed to achieve this.