Thanks for your question. I will try to help.
If the property is a guesthouse at the moment the you are correct in that it is use class C1.
Unfortunately there is no permitted change to another uses class (including residential use) without a planning permission being obtained from the local planning office.
This means that until such time as a planning permission is granted by the local authority authorising a change of use to another use then the property may only be used as a guest house.
In theory, there would probably be nothing stopping you from buying it and simply holding it unoccupied but this would not change the use of the property. There is no length of time that you could hold it unoccupied for which would then allow you to use it for residential purposes.
Therefore, if you propose to live in the property then you would have to make a planning application for change of use to residential. It would therefore not be prudent to buy the property without knowing whether this is likely and for this purpose it would be sensible to consult fully with a reputable local planning consultant before you buy it.
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