Replacing fascia board or soffit will not normally need building regulations approval.
You are advised to check that the replacement work does not reduce the ventilation provided to the roof void as this could cause condensation to occur within the roof void, which can then lead to damp on the timbers. If the existing system has vents installed a similar system will need to be maintained.
The replacement of fascia and soffit boards (as described below) will not normally require approval under the Building Regulations.
Fascia boards - these are boards attached to the end of the rafters/truss at the eaves, where the guttering is attached for the roof rainwater drainage. These are also placed at the ends of gable roofs to cover the rafters/truss.
Soffits boards - these are placed on the underside of the eaves where the roof overhangs the walls. This is where the ventilation holes are generally provided.