Part L of the building regulations, which applies to the design and construction of new buildings and the reconstruction of existing buildings, sets out the legal requirements to be observed in relation to the conservation of fuel and energy.
Technical Guidance Document L, which accompanies the regulations, provides guidance on the thermal performance standards of various elements in buildings, including roofs, walls and so on. For typical construction, the technical document provides guidance on determining the thickness of insulation material required to achieve the standards, taking account of the thermal properties of both the insulation and the basic construction.
Responsibility for compliance with the building regulations is a matter for the designer, builder and owner of a particular building. Local building control authorities are responsible for enforcement of the regulations. It is open to any person to contact the local building control authority about alleged non-compliance with Part L of the regulations in specific cases. I have not received representations about any general problem of non-adherence to Part L.