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Thanks for replying. Sorry should have made that clear, it's one full width dormer at the back of our flat, although one half of the dormer is not going down to the very bottom of the roof. That's why I thought it was so weird that the room height the dormer creates hasn't been fully used as indoor space. I'll look into it because it would be very nice to have the additional space.
Can't tell if you mean there is one full width dormer or two separate. If the latter - if it's in a conservation area or a listed property (?) the LA may have required a less dominant / more appropriate roof design for the property. Out in suburbia it is absolutely de rigueur to add a full width dormer box on the back of victorian/1930s terraces and semis using permitted development rights. So in principle totally possible subject to planning/ structural/building regs.