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.
Nobody wakes up one day and says, "I need extra ventilation", without good reason. Ventilation systems are usually ordered to solve a problem, or to prevent a problem before it occurs. We can fix condensation throughout a property, poor ventilation and removing bad smells in kitchens and w/cs - we can't fix water ingress, leaks or rising damp with ventilation alone.
