Building above existing ground level dwelling

Our property has an annex to the side with a flat roof which we would like to build a room on. Different builders tell us different approaches to do this. Two of them say the whole flat roof has to be taken out first and the building won't be water tight until the room on the upper level is built and roof put on. One builder say he would build the room on the upper level first, then work on the flat roof (floor of upper level). This sounds the least disruptive but is this possible? I mean will the builders be able to build on existing flat roof first? What if they need to change the joists?

if the existing flat roof joists are unsuitable or undersized then they will need either replacing or sistering with bigger joists alongside = roof off open to the weather.

if the existing flat roof joists are adequate but at the incorrect (floor) level then they will need raising = roof off open to the weather.

if the existing flat roof joists are the correct size and are at the correct height then it is possible to construct a timber inner frame and even the roof prior the the outer skin being built = this will mean cutting into the roof but at specific points close to the perimeter.

at no time, early in the project will your roof will ever be completely weather tight unless you construct an overhead canopy scaffold.

I would say roof off but see no reason why the ceiling shouldn't be able to be left intact, unless the existing roof joist are higher than the desired floor level.any decent builder should be able to keep the waether out whilst carrying out the works