The short answer: often, yes, and only your local planning authority can tell you which. Turning a flat roof into a space people stand on changes what the building does to its neighbours, and that is the thing planning exists to consider. This page sets out what to check, not what will be decided.
Why is a terrace treated differently from a flat roof?
Because nobody stands on a flat roof. The waterproofing on a roof affects only the building it sits on, whereas a terrace puts a person at first floor level or higher with a view over whatever is around them, and that is a change in how the building is used rather than how it is finished.
That distinction explains most of what follows. A re-covering is a maintenance question. A terrace is a question about the relationship between your building and everybody else's, which is a different department and a different conversation.
What is overlooking, and why does it decide so much?
Overlooking is the loss of privacy that a raised outdoor space can create for the people around it, and it is the single concern that comes up most often with roof terraces. A terrace sees into gardens and windows that a roof never did, and unlike a window it has no fixed direction of view.
In practice this is why so much of the design conversation ends up being about screening, balustrade type, which edges are usable and which are set back from a boundary. Whether any given arrangement satisfies your authority is not something a roofing contractor can tell you, and anyone who says otherwise is guessing with your money.
Does changing the look of the building matter?
Frequently, yes, and it catches people who assumed a roof terrace was invisible from the street. Balustrades, screens, access structures and raised parapets all appear on the outside of the building, and a proposal that alters what the building looks like from a public place is assessed on that as well as on privacy.
It also means the terrace and the way you reach it are one proposal rather than two. A door or a stair formed to reach the roof is part of what is being considered, and it is worth having that drawn before any view is taken, rather than treating access as a detail to resolve later.
Does it change anything if the building is flats?
Yes, in two directions at once, and this is where people most often come unstuck. Planning is one gate; the building's own ownership arrangements are another, and clearing one has no effect whatsoever on the other.
In a block, the roof is very often not yours to build on even where a council would permit it, and permission from whoever owns the freehold, or from the company running the building, is a separate application on separate grounds. The general shape of who owns and maintains what is covered in the guide to roof responsibility in a leasehold building, and your own lease settles the specifics.
What about conservation areas and listed buildings?
Both raise the bar and both are worth establishing before anything is designed. In a conservation area the external appearance of the proposal is weighed against the character of the area, and on a listed building work affecting it is a separate consent altogether from planning permission.
Neither means the answer is no. Both mean the question is asked more carefully, that the design is likely to matter more than usual, and that professional advice at the front end is money well spent rather than a formality.
Is planning the only approval involved?
No, and the two are routinely confused. Planning is about whether the change is acceptable in its setting; the Building Regulations are about whether the construction is sound, which covers guarding, access and the structure carrying it all. A scheme can satisfy one and fail the other.
Which of the two a given project engages is worked through in a separate piece on which roof jobs the regulations reach. What matters here is only that permission granted is not construction approved, and that whatever a designer signs off for the job is what the specification eventually records.
Who should you actually ask?
Your local planning authority first, and a planning consultant or architect second if the answer is anything other than straightforward. Those two decide between them what is possible; every other party in the process, ourselves included, is working out how to build what they have agreed.
We are clear about that boundary because the alternative wastes your money. What we can answer, and answer before you spend anything on drawings, is the physical half: whether the structure will carry a terrace, whether water can still be got off a roof with something standing on it, and how much height the existing door leaves you. Any one of those can rule a terrace out regardless of what a council would have said.