All new build and most structural alterations require a Building Regulation application for the work. This applies to removal of load bearing walls, new openings, alterations to roof structure, including the removal of props or the addition of strengthening which may re-distribute loading. There is a very good reason for this in that a building may function satisfactorily, but once loads are redistributed to different locations, problems can arise. It is always advisable to write to the local authority and obtain written clarification of whether an application is required.
Alterations where items of structure are being repaired like for like do not usually require an application, but again it is best to check first. Re-roofing does not generally require permission and if decayed roof timbers are being replaced with timbers of equal quality and size in exactly the same place as they were originally, then again this does not normally require permission. This also applies to the re-building of parts of walls and other similar operations. Note insulation requirements though.
A professional in the field of the Building Regulations should always be consulted before works are carried out. A Structural Engineer can advise on what works are necessary to comply with the Building Regulations, particularly where structural alterations are being undertaken. The Building Regulations are there to protect you and subsequent owners, not as a hindrance.