I suspect that they are not allowed to turn it into a home due to daylight and ventilation requirements. It certainly can’t be rented out. So instead they sell it as a basement, and let the owner fix it themselves. Meaning that the property develop just got 30k for an otherwise useless basement.

For example: https://www.righttolightsurveyors.co.uk/sunlight-and-new-house-design-beware-the-temptation-to-use-standard-plans-without-full-consideration/