Any purpose built or converted Annexe will likely need planning approval and it will be easier if the Annexe is to be ancillary to the main dwelling, ie for use of the family. However just like any planning application, it will very likely be down to the individual planning case officer to give approval or refuse. Common reasons for refusal relate to the likelihood of being easily able to separate into a separate dwelling in the future and the impact or the existing dwelling, garden and neighbouring properties.
Further investigation should be made to your local authority Council Tax department if a member of the family is using the Annexe full time, although you may be allowed discounts depending upon the occupier. The Annexe will need to meet all usual Building Regulation requirements.