Building Regulations: Work to an existing roof
If you want to carry out repairs on or re-cover less than 25 per cent of the area of a pitch or flat roof, you will not normally need to submit a building regulations application. You will need approval, however, if:
You carry out structural alterations
The performance of the new covering will be significantly different to that of the existing covering in the event of a fire
You are replacing/repairing more than 25 per cent of the roof area, in which case, the roof thermal insulation would normally have to be improved.
The removal or alteration to any roof elements could affect how the roof works and cause movement to occur. Movement could cause cracks to occur in the walls and, possibly, the eventual collapse of the roof. When performing work on any roof, care should be taken to ensure the roof will continue to perform effectively and without any movement.
https://www.planningportal.co.uk/info/200130/common_projects/47/roof/3