UK Spouse Visa - Living with Parents

Planning on submitting my fiancée's spouse visa near end of March 2020 (getting married early March). My fiancée is from USA. We'll be submitting 6 pay slips for myself showing earnings of £18,600 from overtime/bonuses, pictures of subsisting relationship, marriage cert., bank statements to corroborate with pay slips.

The accommodation requirement. I don't want to get an apartment until we are in the UK together and my parents have said we can reside with them until we're able to move out. They pay a mortgage but I am unsure what documents I need to provide to meet the accommodation requirement. Is it X amount of bank statements from my father's account to prove he is paying a mortgage, with a letter confirming we are able to live there with them? It's a 3 bedroom house with just myself and my two parents living in. My sister receives mail to this house although lives with her boyfriend most of the time. I'm guessing HO would consider 4 people to live in this house (myself, sibling and two parents).

Any advice would be great.
Thanks.

A copy of the land registry and a letter confirming that you can both reside should suffice.