julian said:
Trussed-rafters are a kind of immaculate one-hoss-shay: they can provide large spans out of small-dimension timber, but with zero redundancy. You can't remove any of the members - so to make space for an extension upwards you need to 1) replace the entire ceiling / new upper floor throughout the bungalow, because the lower horizontal members are nowhere near stiff enough to support a floor on their own, and 2) then build an entire new structure to support a new roof... Move house in preference - it would be a lot cheaper & easier.
With the traditional approach of rafters spanning from walls to ridge with an intermediate purlin or two and maybe a couple of diagonal braces in the middle of the whole loft space it's a lot less hassle to make things work. If your house was built more than 20-25years ago, this is probbaly what you have.
Seeing as you have your aunt&uncle to hand I'd get them involved good & early.
Discusison of the best way to detail the upper floor/ceiling/isolation has to follow-on from working-out how big the basic structural problem is - and this involves review of the walls downstairs. But to give some hope - and of course depending on how much upheaval you can stand!- it may be possible to retro-fit a concrete-based upper floor if thats what you decide you want, but it's not necessay to good isolation and in-room response.
(edit: Julian - I'd be happy to answer detail tech queries as best I can via PM if you like.)
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