Not a bad rule to use the last quarter as a baseline. Might limit drastic increases in rates.
Can’t say if this really helps with consumer protection, especially if you fix for long duration.
A few other points on French mortgages though:
- 75%-80% max LTV seems to be standard
- 10-15% of property price in transaction fees not unusual
- total mortgage term payments not more than 30% of household disposable income
- you must wait 10 days before accepting a mortgage offer (enough time to read the small print, imagine that!)
- some cities provide interest-free credit to help with the down payment if you don’t have enough savings