Some repairs to flats roofs will not require an application for approval under the Building Regulations. However, if the roof with integral insulation is to be replaced then you may be required to upgrade this 'thermal element' of the structure and reduce the amount of heat that was originally lost, by upgrading the insulation.
If the existing roof covering is to be replaced with a different material to its original for example, slate to tiles, then approval under the Building Regulations is likely to be needed to ensure the roof will be adequate in terms of structural stability (applicable where the replacement tile will be significantly heavier or lighter than the existing), and also meets requirements in respect of fire safety and energy efficiency (see above).
If the new roof covering is significantly heavier or lighter than the existing one, the roof structure may need modifying and/or strengthening, and you are advised to check with a structural engineer or surveyor before commencing with works.
As a roof is defined as a thermal element, the work to re-cover a roof should also include for improving the thermal insulation properties of the roof.