'Annex' is just a term and it does cause confusion. In short, in planning terms, there are three categories of accommodation which could be contained within an outbuilding:
1. 'Incidental' to the main dwellinghouse- these are functions like a games room, garden room etc, but would not include any primary functions which would normally be expected to be found in the dwellinghouse such as bedrooms, bathrooms, kitchens. Outbuildings serving this function can potentially be built without planning permission.
2. 'Ancillary' accommodation- such as a guest bedroom with en-suite. An outbuilding containing such rooms would always require planning permission but does not form a separate unit of accommodation in its own right. Those residing in the outbuilding should retain a demonstrable dependency upon the main dwelling and remain part of that household.
3. Separate units of accommodation- a self contained unit of accommodation with all the facilities required to function separately to the dwellinghouse (i.e. bedroom kitchen bathroom)- this would be considered a separate planning unit and would require planning permission as such. Whether it is proposed that a family member would live or not there is not really the determining factor, as even family members could live separately to the main house, and whilst the initial occupant may be a family member this will always be temporary, at some point circumstances will change and what will be left is a separate unit of accommodation.
The term 'annex', is often used when someone is proposing an outbuilding / accommodation which falls in to the third category, but they would like the Local Planning Authority to consider the proposal as something in the second category! Some LPA's will do this and may apply conditions to restrict the use of the annex to try to avoid its use as a separate unit of accommodation. Others will require that the proposal is applied for as a separate unit of accommodation.
The simplest form of annex is one which actually falls into the second category; e.g. a side extension to the dwelling with contains a bedroom, living space and bathroom, with the occupant still sharing use of the main kitchen of the dwelling and remaining part of the household. As soon as an annex is proposed in an outbuilding, it may be considered a seperate unit of accommodation, particularly if it does include a bedroom bathroom and all other facilities required to function separate to the dwelling.