Many landlords with HMOs (C4's in planning terms) applied to Portsmouth Planning dept 10 years ago to get C3/C4 status – meaning they could freely switch between family use (C3) and HMO (C4) without needing planning permission.
At the time, they were not told that the C3/C4 flexibility only lasted 10 years and none realised the implications at the end of the period – but based on comments from Portsmouth Planning this month, many long-term HMO (C4) landlords risk losing their C4 status at the end of the period with no chance of getting it back.