Most residential new-build properties include a cooker hood over the hob. It's the accepted 'norm'. It comes as part of the Kitchen Fit-out package in a house or apartment build and its specification is usually dictated by form not function!
Modern cooker hoods can be set-up to either re-circulate air through grease and carbon filters, returning the filtered air to the room, or exhaust via a short duct run to atmosphere.
The cooker hood straddles the line between a domestic appliance and a piece of ventilation equipment. Used as a re-circulating hood where air is returned to the room it does not satisfy either Part F of the Building Regulations or the Domestic Ventilation Compliance Guide as a kitchen extract fan. In this mode it's just a domestic appliance.
Used as an extract hood with its own exhaust duct it has the capacity to cause a negative pressure by extracting more than air supplied by air infiltration or any type of mechanical system!
If you have a open plan kitchen diner with a log burning stove it is not advisable to install a cooker hood extracting directly outside due to the possibility that carbon monoxide could be extracted from the stove, This is still the case if the stove has or does not have its own external air supply
The ideal scenario is to treat the hood as an entirely separate, occupant operated, filtered recirculation device to capture grease and reduce cooking smells whilst the Continuous Mechanical extract system (MEV), or MVHR system operates in the background to ventilate the property as Part F intended.
The worst scenario possible is to duct the extractor hood in series with the MVHR or MEV. A non-running extractor hood or one with dirty filters provides high resistance on the kitchen leg, preventing the central ventilation system from extracting the correct amount of air from the kitchen. A running, high air-flow extractor hood pushes more air into the central system than it can extract away, resulting in kitchen smells being blown back down the extract ducts into other wet rooms!
If you or your client decide you absolutely must have a kitchen extract hood in addition to an MVHR/MEV system then your only option is to use a hood with a filter, light and boost switch but no fan. These do exist!
The filtered hood can be mounted above the cooker in the normal way and ducted direct to either system. The switch is wired back to the MEV/MVHR taking the unit to boost when required. Occupant control is far less confusing and correct ventilation rates are assured.