There needs to be a clear minimum size for a room to be classified as a bedroom in rental properties. There are minimum sizes for HMO's but not for other properties. I would suggest 9sqm is a fair amount to have a bed and storage.
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The private rental market is ridiculous and too many people are taken advantage of. In London particularly, tiny spaces are being marketed as bedrooms and people with a budget are forced to deal with it. In a market which is already dominated by the landlords, please at least ensure that there is room for someone to walk in their own bedroom.