I think the date is fairly arbitrary - we calculated 3 months from our Visa application as looking at processing times we were fairly certain we would have a result at this point...
From an accomodation perspective I submitted a copy of the deeds for the house (you can get a copy online) , a couple of utility bills and council tax bill as proof of address and also the original estate agent brochure for the house which stated the room sizes and included pictures. I also made it clear in the covering letter that the house was wholly owned by myself and that my wife and I would be the only residents in the house.
Hope this helps - we were approved a few days ago so feel free to ask any questions regarding what we submitted and how we submitted it.
Steve