You're planning to put your joists from A-B and B-C and holding up the
roof some other way. Most loft conversions I designed in my earlier
life had serious steel beams at D and E spanning from party wall to
party/flank wall: these supported the floor joists which now only have
span D-B and B-E also the dwarf partitions that help hold up the roof.

I see now.

This could possibly work for me at the front. However, the back is planned
to be a full dormer extension, so this wouldn't work there. I'd prefer the
floor to extend from A-D for storage purposes, although a lip from a beam
would be no great concern. Unfortunately, the BC joist still needs to span
the distance, without the E beam, so it doesn't solve the rear problem.

Would you still recommend hanging the front joists off a 'D' beam, or would
it be more economic to just span the lot and build the partition across the
joists? Note that an escape window will almost certainly be required that
will probably require floor space forward of the beam. I want at least some
light storage space in front of the beam, including a compartment in the
escape window "tunnel" to hold a rope ladder and other safety equipment.

One additional complication is a full height bay window at the front of the
property. This could cause problems finding somewhere for those joists to
hang from. Obviously I can have a beam of any height at that point, provided
that the joists can hang with its bottom in the same space and not sit on
top. This is effectively the 'D' beam solution moved so the beam is at the
front edge of the wall, but using steel joists, rather than wood to span the
gap. However, is it possible to hang or weld steel joists off a beam in the
same way as timber?

/ / - rafter ''
/ / stud partition -- ''
/ / ''
/ /+----+ ''
-- |beam| -- weld/joist hanger? ''
##| +--------------------------------------++-----
##| | 90x90x3.6 SHS Joist ||
#####+----+--------------------------------------++-----
------------------------------------------------+--+---- p/b
| |
bay masonry wall - | |
extension | |

(existing wood ceiling joists not shown for clarity
Christian.