The fitting of skylights does not normally need planning permission and falls under the permitted development rule but is subject to restrictions: New additions cannot have more than 150 mm projections above the existing roof and obscure glazing must be use on side facing windows.
The re-covering or structural alteration of a roof greater than 25% will require filing a building regulations application. This type of work must comply with the Approved Document L 1986 which sets out to increase the thermal performance of existing buildings. In this particular case, the roofing insulation must be upgraded to the require u-value. The u-value is the rate of heat lost through an insulating material and the lower its u-value the better the insulator.
Good ventilation of the roof structure will prevent condensation damage which can be achieve by a combination of methods: The fitting of a breathable felt below the tiles, fitting ventilated ridge tiles or ventilated tiles or installing vented soffit boards.