On the face of it, it seems a fairly straightforward question that should have a straightforward answer but when it comes to marketing your property for sale, there are certain regulations you have to be aware of before it can be counted.
Generally speaking a bedroom can only be called a bedroom if it has a window, is up to current building regulations (or the building regulations at the time it was built/converted) and is of a sufficient size. Examples where this would not be met is a loft conversion without standard stairs to access it, a room with only an internal window using borrowed light or a room you simply couldn’t put a bed in. We recently had a property to let that you could fit a cot in but a bed would have almost filled the room so we made the decision to not call it a bedroom, to avoid misleading anyone.
It is possible to call a dining room a bedroom if in common usage it would be used as a bedroom even if the current occupiers don’t use it as such. It would only be possible to do so if there were no connecting doors or access to other rooms though. For us most of the time it’s about the balance of the house, there is no point in having 6 bedrooms if that leaves you with a small lounge, a kitchen and a bathroom.
That’s not to say that you can’t make the most out of a very small room with some very clever interior design, just take a look at what Ikea do with their studio flat layouts in store.
If you have a very high ceiling in the room there is always the possibility of using a cabin bed with wardrobes etc underneath. Another alternative is to steal a bit of extra space from a neighbouring room, it doesn’t have to be as complicated or expensive as it may at first seem. Just be careful not to be left with 2 single bedrooms instead of a double and a study or a nursery!