The limiting factor will still be the household budget.
Parental allowance will be around €1000, child benefit will also be added.
This leaves a net amount of €2900.
Banks calculate a household flat rate of about €1500.
Additional costs for the house are about €350 / month.
This leaves about €1000 for the installment during parental leave, but you will not fully use this.
Let's assume actual construction costs (including additional costs) of €350,000.
Equity = €100,000 + land value (I assume €50,000 for this example calculation).
Total volume = €400,000, financing requirement €250,000.
This results in a monthly installment of €753 with an interest rate fixed for 20 years and an effective annual interest rate of 1.53%.
Remaining debt after 20 years at a manageable €133,000.
It will be interesting to see how things continue after the parental allowance period. If your wife then goes to work part-time and, for example, only earns €850, this is still financially feasible, but there is not much room for maneuver.