I tried this approach but all the lenders I tried wanted the whole property as security for the loan.
It is hard to split a property for security purposes. Imagine if you do not pay the loan and then say UBS want to sell their part and CS want to keep their part.
You can sometimes do it. It means having more than one "Schuldbrief" (security issued on the property). It is best done at the time you take out the initial loan because the holder of the Schuldbrief will be reluctant afterwards to allow it to be split.
And kinda confirms my fear that by signing up partially to a long term deal with one lender, even if only say for 30-40% for 10yrs, means you are generally stuck with them for the whole 10yrs, even when your other shorter tranches expire.
And I guess when their great rate for 60%-70% @ 5yrs they offered you is finished, they wont be so willing to do a great deal for you, when they know they have you anyway for another 5yrs on the 10yr deal.
I have 3 tranches with different term dates. I have not found this to be the case when re-negotiating the tranches. If you think the rates are going to rise, you can forward fix the rate up to 24 months in advance.
When negotiating the rate, it is better to negotiate a margin on the swap rate. That way it is possible to keep the same terms when negotiating a new mortgage.