Carpenter's loft conversion headache

[CavanaghDuBois]

Been asked to prepare a loft space for a future Dormer conversion. Its a bungalow with single span, 47x100 ceiling joists. my question is; How do I manage the depth difference between the flooring joist needed and the existing ceiling joist? Do I need to replace the joists completely or is there a trick I can use that stays within building regs.
new joists will be at least 170mm deep, maybe even 195mm depending on the span

 

[blup]

There is little you can do practically other than clear out the loft space IMO, the size and location of the new joists will depend on the design and structural requirements of the conversion, you might even need steels. There is no getting round the need for building regs approval.

Blup