There is no specific law in the UK that makes it illegal to leave a person with dementia alone. However, the picture is more nuanced than a simple yes or no. Under the Mental Capacity Act 2005, decisions must always be made in the best interests of a person who lacks capacity. If leaving someone alone puts them at significant risk of harm, it could constitute neglect, which is a criminal offence under the Care Act 2014 safeguarding provisions. Family carers who repeatedly leave a person with advanced dementia unsupported in an unsafe situation could face safeguarding investigations. The practical question is not simply legality but whether the person is safe. If there is doubt, contacting the local authority for a care needs assessment is the right course of action.