quickymart:
It's not an ideal situation, but if that's what's required to service a mortgage, so be it.
Seems a bit naff a flatmate needs to sign something saying they'll live there however (I saw someone say this earlier in the thread).
Its perfectly reasonable / understandable.
Its come about in large part because of the 'Lier Loans' which caused the GFC.
With the responsible lending requirements now in force, banks need to show they have made reasonable endeavours to prove the borrower has repayment capacity.
Words are cheap - if you have documented evidence, of future tenancy, clearly the banks can prove they have taken further steps to confirm/prove your capability to repay the huge debt you are taking on.