Do you need planning permission to use a flat roof as a garden?

The short answer is 'yes' - by utilising an existing flat roof you are placing yourself into an elevated position which could result in neighbourhood issues with overlook and loss of privacy. Planning permission is needed for any such development to ensure that all potential problems are considered; addressed and suitable solutions found before the roof is used as a terrace.

Previous answer correct for planning AND you will need to consider the adequacy of the roof to accommodate additional loading from people; this will be MORE than the snow load it should have been designed for. You will need a Structural Engineer to survey and assess the existing roof. It is highly likely that the roof will have to be upgraded for the change of use.. Think smart too, new planning requirements ask you to consider 'Blue ( water retaining) ' roofs or 'Green ( holding water thorugh growth)'. So, think about the additional twist, that the change of use can easily be made to be environmentally beneficial too.