Cost of converting a flat roof to a pitched roof to make a storage loft?

[WeBuiltThisBuffetOnSausageRoll]

Does anybody have any idea of how much we might very roughly expect to pay for this? We may be in a position to consider doing it later this year, if the cost is not massive, but I really have no idea at all what kind of cost we'd be looking at - it could be achievable or it could be astronomical.

The flat roof is on a single-storey extension (the main house attached to it is two-storey and already has a normal pitched roof), is approx 28ft long and 22ft from (what would be) the eaves at the front to the eaves at the back - we would probably have it a maximum of 6ft high along the roofline.

Not planning for a conversion as such or for a living/sleeping area - just a large dry storage area with some kind of solid flooring (using the existing roof or replacing it/covering it with floorboards). No electrics needed except a light or two in the ceiling; doesn't need to look pretty - just dry and functional.

Any suggestions for cost or considerations/options very gratefully received - thank you!

Would you need planning if raising the roof line?

I'm not certain, but as I interpret the regulations as I've read them, I don't think we would. The original part of the house (200yo) that the single-storey extension (50yo) is attached to is two-storey, so a roof on the extension wouldn't be anywhere near as high as the existing highest point of the house - and it wouldn't stick out any further than the existing edges of the roof (except for possibly a few inches where the eaves would adjoin to the existing roof, which I believe is allowed).

I don't know for sure that I've interpreted the 'permitted development' rules correctly, though.