How to achieve 260cms ceiling height with flat roof in my extension

There are a lot of factors to take into consideration when looking to have an extension. I am presuming it going along the back wall of the house ? Height your asking about is 2.6m inside, A warm roof is preferred by council if installing a flat roof The outside full finished height would need to be checked against the height of any upstairs windows. A sloped roof could be also considered again there in a minimum angle for the fall/ slope, this would need checking against any upstairs windows. A pitched roof can be a warm too the full height of the pitch is more than a flat roof. My advice would be to get a drawing done by an architect ( they would also be able to offer advice on the regulations and planning) send it into the planning department, they would come back to you with advice, if they refused they would say why . You can then look to change the proposals to follow & resubmit . Use a local builder to help with the initial ideas.