Really depends on if its a flat or a house and if you can get access to the ceiling void. In a house with a first floor bathroom, running the cable in the loft above the bathroom is a lot easier than in a flat where it may not be possible to get into the ceiling void from above.

Ballpark estimate is around £150, for a reasonably good fan, plus installation.

Hello natasha , The cost would vary depending on the style of fan required , distance from possible fan position to outside via corehole in wall , duct to nearest soffit or duct to tile vent .You would also need to ensure that the new installation was rcd protected as it would be classed as a special location . So lets assume that everything with regards to additional protection ( rcd , bonding , main earth , etc ) was sufficient and this was a simple extract to a soffit 3 m away then you would require a centrifugal fan with timed overrun . I would give a rough estimate of £165 for this to be completed .

There are alot of variables to a job of this nature. An extractor with a humidistat is always an option if your bathroom is prone to damp caused by condensation build up or an extractor with timer overun which you can set to suit your needs. I would advise you to call a local Part P registered electrician to pop round, assess your current wiring and fuseboard and provide you with a quotation. A bathroom is deemed a special location, so any circuit alterations will need to be notified to building control and a certificate issued directly to you.

Good luck.

this would depend on whether you have an rcd controlling the light circuit,where you plan to vent the extractor(through an outside wall/ceiling mounted vented out to a soffit/ first, 2nd floor etc),what part of the country regards Terry.