Building Regulations (Part L1B) sets out the energy efficiency requirements where more than 50% of a thermal element (see below) or more than 25% of the overall building envelope is being renovated.
A major renovation means the renovation of a building where more than 25% of the surface area of the building envelope undergoes renovation. When assessing whether the area proportion constitutes a major renovation of a building, the surface area of the whole of the external building envelope should be taken into account i.e. external walls, roof, floor, windows, doors, roof windows and rooflights.’ (Approved document L1B pg. 17).

In the case of walls, the thermal element refers to a wall that separates the heated part of the building from the external environment.
This means that all external wall insulation is therefore notifiable with Building Control. It also means that whatever renovation you are undertaking to your wall, if it affects more than half the wall (or more than a quarter of the overall building envelope) you will need to add insulation to improve the thermal performance of the whole of that individual thermal element.
The Building Regulations (Part L1B) requirement is to ensure walls have U-values of no more than 0.3 W/m2K. (The U-value is a measure of the flow of heat through a thermal element. The lower the U-value the better an insulator it is).