So does this mean that you cannot rely solely on an operable window to vent your bathroom even if it exceeds the miniumum 1/20th of floor area?
That appears to be the implication from TGD Part F 2009 Table 1 & Diagram 4.
The mechanical extract is a requirement for all new dwelling bathrooms.There is an emphasis in these regulations on moisture extraction.
This extends also to Kitchen areas where the built-in cooker filters which recycle the air are not considered adequate in terms of ventilation and vapour extraction (Ok, this is my opinion only, based on recent research). A mechanical extract to the open air (or passive stack system) is generally required in new kitchens.
Para. 18.104.22.168 says "The appropriateness of a particular fan for a particular use should be verified by reference to manufacturers data" and then specifies the required tests.
If anyone has conclusive evidence that charcoal filters over cookers satisfy the Part F requirements, I'd certainly appreciate it if they could share this knowledge.
(This is important when it comes to certifying houses/extensions for compliance with the Building Regulations).
You can always try another approach apart from the TGD to provide evidence of compliance. Maybe an M&E Engineer might advise.