On 15 Feb 2004 11:54:03 -0800, a particular chimpanzee named
(Chris Allmark) randomly hit the keyboard
and produced:

I am looking at converting my loft (3 bed semi) into a bedroom.

A hipped roof wants to push the walls out as load is added from above
(wind, snow, etc). This is normally restrained by the ceiling joists
forming a tie from one side to the other at wall plate level, the
purlins and (hopefully) angle ties at the feet of the hips. By
removing the ceiling joists and installing floor joists, the natural
triangulation is removed. Further removing the purlin to install
dormers will create an 'unbalanced' roof.

None of these things are impossible to overcome, but will require the
advice of a structural engineer. As mentioned in another post, your
best bet may be to extend the side to form a gable. IANAPlanner, but
it will v. probably require permission.
--
Hugo Nebula
"The fact that no-one on the internet wants a piece of this
shows you just how far you've strayed from the pack".