UK Landlord Forum / Life

My tenant has resumed his marriage after a separation and his wife has now moved into my flat with him. She has never lived in the property before. It is let on an assured shorthold which expired back in the autumn and the tenant is now holding over. I am struggling to find anything on line about how to amend the tenancy agreement to reflect this change. It is important that I get it right as there is a chance that we may need to sell the flat, in which case we will be seeking possession.

Any advice greatly appreciated - I hope I haven't  left out the most vital information!