Kitchen ventilation issue

[spud1962]

I have a domestic cooker hood with a built in extractor fan
which currently ventilates to an outside wall via about 6ft of
"bog standard" plastic circular ducting (the stuff that looks like
a "slinky spring toy" with plastic coat on!).

I am currently having a single storey kitchen extension built which
means that the outside wall currently used to expel the exhaust, will
now form part of the new roof space/void.

no doubt I could just extend the ducting another 10 foot to the nearest
outside wall in the the new extension, but I think that the overall
length would just be simply too long to cope with the exhaust.

I was thinking of installing some sort of "in-line" fan in the wallspace
at the point where the existing ducting terminates,to boost the airflow,
then continue with the "extended" ducting to the nearest outside wall.

Could anyone suggest such a device capable of this scenario and are there any building regulations to be wary of?

 

[nstreet]

Why not just vent upwards through the new extension roof?

 

[Bylough]

I think such a long pipe would be a cleaning nightmare, and the build up of grease would be a fire hazard. Still, would an external extractor be poss?