Depending whether the foundation for a new detached building has been cast as deep-fill or shallow-fill, there could be a small or large amount of wall construction needed below ground level (referred to as substructure), on which the above ground walls (referred to as superstructure) will be built.
The principal requirement of the substructure is to ensure adequate support is provided to the superstructure. The substructure (bricks or blocks and mortar) should be effective and be resistant to frost and also to sulphates within the ground.
Disclaimer - Wales
This is an introductory guide and is not a definitive source of legal information. This guidance relates to the planning regime for Wales. Policy in England may differ. If in doubt contact your Local Planning Authority.