I want to visit my bf in the UK with a standard visitor visa, he is my sponsor, he can easily pay for all of my expenses, but he lives with his mother and I'm going to stay at their house. Seeing as the owner of the house is his mum, should I add her as a sponsor on my online application too or can she just write an invitation letter allowing me to stay in her place?
Although she is the owner of the house, the mother is not your sponsor. She is not providing you with direct financial support during your stay, i.e. she is not paying for your expenses. However, she will have to be mentioned in the invitation letter. Whoever is inviting you must declare that they do not own the house and provide proof that the owner is informed about this and agrees.