Applying for Non-O Visa and one of the documents required is "Copy of title deed". My landlord asked me why we need this - is this the Yellow Book or something that only need the first page of it? Thank you!

Morning everyone. I'm just applying for a visa and i used my blue book for the apartment I own when I submitted the application as proof of accommodation. They just called me and said that the blue book is not proof but they couldn't advise me what is proof - they asked if I had some other document that said I own it. How do I prove I own the apartment to the consulate apart from the blue book? Has anyone had / solved this issue? Thanks in advance.