Stephen formulated on Sunday :
Hello,

My hob has a cooker hood above it but it has been installed in
re-circulation mode. If I change the carbon filter it might absorb
smells but it will not do anything about steam, so I would like to
convert it to extract outside. That would be straightforward: the hood
is about 6.5' or 2m ish from the wall. I could run ducting along the
top of the cupboard units.

Check which way the joists in the floor/ ceiling above run, you might
be able to fit a suitable duct between joists. That is how I did my hob
extract. Rigid plastic pipe up from hob into ceiling, then duct between
joists, terminating in a gravity closing terminal on the outside wall.