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The rules around a third party providing accommodation support https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/827480/Visit-guidance-v9.0ext.pdf#page21 require that:
there is a genuine personal or professional relationship between the third party and the applicant
if the third party is in the UK, they must not be in breach of immigration law at the time of the decision on the visitorβs application.
No specific document to demonstrate the above is given in the βSupporting Documentsβ guide https://www.gov.uk/government/publications/visitor-visa-guide-to-supporting-documents/guide-to-supporting-documents-visiting-the-uk Typically, the requirements are fulfilled by the host providing the applicant with an invitation letter that explains why they are willing to have the applicant stay in their home, how long the applicant is going to stay, and that they have enough room for the visitor, for the entire period of the stated stay. The host should also provide evidence to prove their citizenship or immigration status in the UK.