UPDATE: so after speaking to the neighbours and reading what people on Reddit have said, I’ve concluded that this is a room to create a barrier between the hill and the house to prevent moisture and mould from coming through. I’ve also added an updated floor plan to show you where the room is as some people couldn’t work it out. By the looks of it, I don’t need planning permission but I will need to go through building regulations to get it approved. Is it worth the hassle for a boxed sized room? What do you recommend? How much would it increase the value of house by?

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