Do I need permission to change roof to different material

if you remove more than 25% of your roof covering you will require building regs if you increase or decrease the weight of your roof by 20+% you may require planing permission and a structural engineers report on how your going to strengthen you roof to cope with the weight difference. different council interpret the rules differently your best bet is speak to your local council building control and ask them there is a lot of different things to consider, what size slate how many slates per m2 compared to how many current tiles per m2 total roof area what is the overall wight difference nobody on here has seen your roof so we have no idea how it is made up currently. good luck Alex