In recent years, part F has started becoming more and more prevalent - and your local building control authority is likely to want to be in receipt of a ventilation certificate and building control notification when an extractor fan is installed.

Aside from that, if you rent, check with your landlord. For homeowners, planning permission generally isn't needed to add an internal extractor fan. However, external wall vents, especially on leasehold properties, may require approval from the freeholder and/or building control.