When is a single dwellinghouse not a single dwellinghouse - Public forum - Planning Advisory Service (PAS)

In my LA area we have lots of historic B&Bs and Guesthouses operating from dwellings, most of which were never formally established through the planning system, but could possibly achieve a lawful status, if required, through a certificate of lawfulness. The question I have is with more recent arrangements, if a person has, for example, put in a separate staircase and created a separate means of access, then it would be more straightforward to argue that the property had been sub-divided into either two units or a mixed use.  If however we have blatant Guesthouses or self-catering suites created (blatant in terms of the advertising or the Council use the accommodation for contractors) , but where the main entrance is used for both the permanent household and the holiday lets, and it is argued is not on a year-round basis (only during the summer tourism season), then this becomes less clear cut.  

The harm is firstly one of consistency, in that such changes, if a person were to ask the LPA before carrying them out, is that we would advise that the changes require planning permission: the change of use of a C3 dwelling to a C4 guesthouse or the change of use of a C3 dwelling to multiple C3 dwellings with the create of self-contained, self-catering holiday lets.  It is less clear when a person has gradually made such changes. So for example initially letting out 1 bedroom on a B&B basis for a few weeks or months of the summer season. This is potentially increased to 2 bedrooms or more (maybe even some family members vacate rooms temporarily to accommodate the income generated). The season becomes longer and maybe even on a year-round basis if the economy allows or the rooms become self-catering or the property starts to promote itself as a Guesthouse and they make internal alterations not subject to planning controls.  This has historically been the case on my LA area. 

My question is when the use of a private dwellinghouse operates some of the time as a different use/mixed use, but the changes made to enable that are all reversible, has a change of use taken place?  I would argue it has if this is a consistent and permanent arrangement that takes place on a regular basis. If I advise that planning permission is required what exactly are they applying for.  It wouldn't be a straightforward change of use from a C3 to a C4 or from a C3 to multiple C3 dwellinghouses, if they are not physically separate. So would it be that an application is made for the very specific 'sui generis' use for the very specific part of the year (quite inflexible).

Has anyone had to deal with this or similar issues?