I can try to help a bit, my fiance wrote a letter saying how we met when we met, we intend to marry, he knows I am applying for the visa and he supports it, and approx when we want for me to be there. He also mentioned we will be staying with his parents.
His mom wrote a letter stating we are to be living with them until the wedding in july, how big the house is, that they own it etc. She also enclosed a certified copy of the mortgage papers.
Hopefully that helps you some!
Im wondering myself what to include in my cover letter when I send out my application.
Good luck! HTH
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Katrina