Ventilation in the Bathroom

Dear community,
If a bathroom has a window which can be opened for ventilation, can anyone advise if it is a legal requirement to STILL have a FAN assisted vent in the bathroom, venting to the exterior of the property?
Any advice is gratefully received - I thought it was not required if a window exists, but a tenant is claiming it IS a legal requirement. are they right?

you need a fan. the tenant is correctthat a fan should be fitted Without one a building inspector will not pass a job or a council inspector will require you to fit one if you need a licence, .fit a 100mm humidity fan comes on automatically Kitchen also should have a fan