Answer: Almost every proposal for a loft conversion requires the modification of the existing roof structure to remove any obstructions within the proposed loft space.
Make sure you a competent structural engineer to carry out the design... This is REALY IMPORTANT!
The photo above shows a 'fink' type truss, these are normally found in modern properties, where the existing common rafters have been strengthened - by fixing a larger timber along side to prevent excessive sagging once the struts are removed, and intermediate volumes are installed, in the form of a dwarf-wall, to transfer the roof load to the supporting steel beams. These steel beams also support the independent timber floor joists.
When designing the steel beams consideration must be given to lateral torsional buckling as the beam is unrestained and potentially allowed to twist when loaded.
Best of luck! Advice on Structural Engineering for Loft Conversions