Hi Werby. It depends on the sclae of the project and the size of the roof terrace. You can get more info on where and when planning permission is needed from the PlanningPortal website. When converting roof areas to liveable space, the roof will need to be checked for adequacy in terms of weather resistance and thermal insulation. Probably the cheapest, and mosty hassle-free way of achieving a roof terrace would be to purchase one of the VELUX roof terraces. They basically work like a skylight but transform into a roof terrace - definitely no planning permission needed here and probably a lot cheaper than doing a ful-on conversion.