The Minimum (legal requirement)height for a gas appliance is 650mm from the pan rests, But I would recommend 750mm if it can be acheived. Most glass or stainless steel back splashes are pre made to 700 or 750mm in height,so its usefull to bear this in mind.The cooker hood/extractor unit will give a recommended height which should be used as a minimum.

minimum height for gas is 750mm and similar for electric,depending on your budget a stainless steel hood with built in extractor wuoldcomplement your range,regards Terry.

Hi, extractor fans are required to be fitted between 600mm and 750mm above a hob, electric or gas so considering a average 900mm high of cooker this would be perfect. As for putting an extractor in a chimney I have seen this before but I imagine that the chimney would not be able to serve any rooms on higher floors and that you would have to run an internal flue or pipe right to the top. Hope this has been helpful. Ed Haynes.

Minimum hight of a chimney above a cooker is 700 mm from the cooker.
Cooker will be approximately 900 mm off floor and the recommendation for the extractor should be a minimum of 700 mm from the cooker.
The extractor size should be determined by the size of the kitchen and cooker, building regulations will have the requirements of that for you.
Hope that helps.