Can you finish a downstairs WC extractor fan in the floor void?

yes ..no problem at all. if your joists are narrow then you may need a narrow rectangular ducting to fit inbertween your ceiling and upstairs floor. edd

NO!

no you would build up condensation in the floor and this could have an effect on the timber in that area ,you should try to run a pipe from the extractor to the outside of the building or through a brick wall after cutting access through it ,,kind regards richard taggart

PARDON???????the whole idea of an extractor fan is to fistly disperse of the moisture content in the air caused by hot water/showers ect secondary smells all of which rise hence the extractor goes to the highest point possible either on wall or cieling and then vent outside ,never heard of 1 venting in the floor,

Totally agree with substructure and richard taggart.Would like to think that ews has misread/understood the question lol.