I talked to Ubs about mortgage this week and you can pledge your 3rd pillar if invested in their account and invested in UBS vita 100 fund, for example. So if you decide to buy in 5 years you would just need to transfer your VIAC to more expensive UBS.
They can also take into account 2-3% of your other invested assets as income for affordability calculations
There are good comments from cortana in this thread