Building a house - building savings contract with bad interest rates

Hello,

my husband and I want to build... We are still quite young (I am 26, he is 31, son 1 and another baby planned in three to four years).
Equity: €25,000 including what we have paid into [Bausparverträge]. We have a mature [Bausparvertrag] (from 2006 with bad interest rates, but good credit interest rates), a [Bausparvertrag] about 2 years old, and a brand new one.

We have found a great house, it is supposed to be built in Ahrensburg by the company "xyz". It costs €360,000 plus the real estate transfer tax plus kitchen. ([Kfw 70])
We have not yet had an appointment with the company but we really want to have it...

Is that totally unrealistic?

Quite simple:

with 1000 EUR net family income = you are crazy

with a few thousand net family income = doable

cost about 400,000€, resulting in a loan of at least 375,000€. With a loan fixed for 25 years and 2.5% interest, you would need a monthly amount of: 1680€ so that it is paid off...

What do you both earn net?

Oh God, Schemelino
My husband works 30 hours at €16.20 (€2106 gross) and I will work 24 hours at €15.45 (€1606 gross) starting 1.8.
I apply every evening for an interesting position that promises significantly more. I don't really believe it will work, but you can always apply
I hope our parents/grandparents will still chip in something that can be paid back sometime...

Sorry, but you can't be that naive...