Land available but only a condominium?

Have you already contacted the general contractors positioned more in the affordable segment for offers (I am thinking of Danwood or Town & Country), just to get a better estimate of the price range for the house?
Do you get better loan conditions if you perhaps partially mortgage the grandparents' property, would that be an option?

The old building stock absolutely cannot be renovated?
Otherwise, maybe first build smaller with expansion reserve. Small children do not need their own room for quite a while, and later on you can also manage well with less (for example, take a look at Danish floor plans, there the children's rooms are planned with 8 m2...).

So when we were in planning in October

we had stated 5.5k net plus child benefit.

We had loan offers from 3.45-4.7% for a 422k loan.
Installment 1.9k - 2.4k possible.

Since then, it has been somewhat higher for me and my wife + child benefit.

( have only bought one house and are now about to move in)

Why is renovation not an option? Even the most extensive renovation should cost at most (for the same house size) 300k, right?

Do you get better loan conditions if you perhaps (partially) mortgage the grandparents' property, would that be an option?

That is probably the most audacious thing I have read in a long time! Taking the grandparents into joint liability for one's own financial well-being, risking going down together in the worst case scenario with job loss (meaning: forced auction of the grandparents' house). I would never in my life come up with the idea of mortgaging my grandparents' house and them giving up their secure financial conditions so that I get a better loan. Anyone who suggests such a thing is more than inconsiderate and selfish. Because the grandparents worked for years to have a debt-free house. Threatening that even with the partner's job is just impossible.

salary me 3000 net
salary wife 2300 net

Ok, good... you obviously have to calculate very carefully. Additionally, there is child benefit. I would also be interested to know whether this is already calculated as part-time or not. Childcare costs (daycare or childminder) also play a major role. Some regions are outrageously expensive, elsewhere it is free or only requires payment for meals. Is there a 13th salary or bonuses? 0, 1, or 2 cars? All of that matters.

Without details, it can also be reduced to current cold rent + current savings rate - €200 higher additional costs = maximum annuity possible.
Part-time work is very important to consider; parental leave personally would not matter to me, you can use a few thousand euros of reserves for that.

What did the bank say about your maximum amount they would finance for you?

And we talk a lot here about a potential inheritance: when and how much can be expected? And how certain is the inheritance?

Ok, good... you really have to calculate very precisely. On top of that, there is also child benefit. I would also be interested in whether that is already calculated as part-time or not. Childcare costs (daycare or childminder) also play a big factor. Some regions are outrageously expensive, elsewhere it is free or only a meal fee. Is there a 13th salary or bonuses? 0, 1 or 2 cars? All that plays a role here. Without details, you can also reduce it to current cold rent + current savings rate - €200 higher additional costs equals the maximum annuity possible. Part-time is very important to consider, parental leave would personally not bother me, for that you can use a few thousand euros of the reserves. What did the bank say about your maximum amount they would finance for you? And we talk a lot here about a potential inheritance: When and with how much can it be expected? And how certain is the inheritance?

The bank does not grant more than ten times the annual net income as the maximum loan; if only one salary remains, then that is €300,000 loan + equity.