Bolted wrote:
The neighbouring semi has a flat roofed extension, but I would very much
like to avoid this from a practical perspective as well as aesthetic.

There's nothing wrong with flat roofs if they're done and maintained
properly. (Well, I know ....)

The extensions will look much better if they are roofed as a pair (I
assume they will share a party wall but not necessarily have the same
ground plan) in a material harmonious with the main house roof.

I think the best thing here would be to have a mansard-style
slated/tiled steeply pitched perimeter roof around both extensions,
visually tying them together, with a central flat area. This will avoid
the height problem of a full pitch roof, and only require a moderate
superstructure over the neighbour's structural flat roof. It will also
work with a complex ground plan. If your neighbour does not want to
extend (!) his kitchen out to meet yours, the plan of the roof could be
taken out as far from the house as yours, to give him a covered external
area which could be 'shedded' as non-habitable accommodation at a later
date.

Internally the kitchen could have a coombed ceiling up into the mansard,
giving extra internal height, or a suspended ceiling giving increased
insulation, downlighters etc. The mansarded part of the roof might be a
convenient place for extractor fan outlets or boiler flues, using
apprpriate terminals, taking them above head height and reducing their
visual impact.

Owain