Void periods are the single largest avoidable cost in an HMO. A room sitting empty for three weeks costs more than the marginal cost of photographing the building properly, and that photography then serves every future void for years afterwards.
There is also a funding argument. Developers scaling an HMO portfolio need to demonstrate to lenders, joint venture partners, and investors that the quality of their conversions is real. A well-photographed completed project is the most direct evidence available, considerably more persuasive than a spreadsheet of yields with no imagery behind it.
If you are converting or refurbishing HMOs in Somerset or the wider South West, the best time to photograph is the week the styling goes in, before the first tenant moves a single box. That window, when every room is complete and nothing is occupied, does not come round again once the building is let.