Buy 変動金利 loan with flat-35

Hi,

I am about to sign a contract for a freshly built house. I don't have PR yet so I cannot have access to flat35 loan.

I plan to get a 40 years 変動金利 loan, and in few years switch for flat-35 (house has highest rankings in terms of earthquake resistance, energy cost for heating, isolation).

Since I work in an "ancestral" 大手企業 my wage increases significantly every year and I expect to clear the loan before the initial 40 years (ideally within 25 years).

I am not very well educated in terms of finances, any comments on whether buying a 変動金利 with a flat-35 can be a good idea or not?