The webpage you linked to is for building control of works, which comes after planning permission. The planning permission bit you need is here. This document gives a good idea of what is usually permitted, although note that some Councils have removed some permitted development rights, so you would still need to check with your local Council. As per that link, some building work would come under permitted development (e.g. small, single storey extnesions or conservatories, some garages, sheds etc). If you are proposing a new building which will be inhabitable (i.e. bathroom, kitchen and the ability to sleep in it) then that would be a new dwelling and is definitely not permitted development. However, if it was purely a bedroom and the kitchen, bathroom and living space were shared with the house, you would need to speak with your local Council to see if they would consider it an extension of the existing dwelling. For example, in East Cambs, they allow any number of caravans on the curtilage of a dwelling provided that you can show they provide ancillary accommodation to the original dwelling and not new dwellings in themselves.
