I'm about to fit an oven hob filter/extractor (of the non-recirculating type) and would prefer to have the outlet exit horizontally but that means it'll goto into a void between my neighbours wall and mine (about 6") which is naturally ventilated at the end (approx 6" x 7 feet thru louvres).

Is this acceptable? Do these extractors kick out much grease and would it be a fire risk?

Is there any other reason why this is not okay?

Alternative is up through a flat roof but thats less than ideal as sooner or later it'll leak and also I've read this style may get wind down draughts..