OK, so far it has been fine to buy a laptop for 2k, a nice trip to the Maldives. What happens when you have to finance 500k and then have kids (net income even less due to possible part-time work)?
How do you repeatedly come up with the absurd idea that +500k€ would have to be financed!?
Even if the old house were kept, there would be additional rent income or you have to explain to us why a house with an estimated value of 340k€ should have a living and/or rental value of zero!?
That is absurd!
Are you willing to consume less, can you handle the expenses and missing net income? I would consider undertaking a bigger project. The banker in me says yes, you can manage it, the average Joe says don’t overestimate yourselves
You are not a banker. The old house has a current and immediately realizable market value of at least 240k€ (70% of the stated purchase price), if the information from the OP is not completely made up. There are 290k€ financed, a small part of which is paid off, including special repayments maybe 25k€, leaving 265k€ - so that can be roughly balanced out in 5 seconds if the house should not be unsellable for some ominous reason.
To break even, you therefore have to get 265k€ plus prepayment penalties for the house.
Only the new house needs to be financed - with the salary and the equity already present (currently invested in the house!) that should not be a problem. At most a technical one, because there will have to be interim financing.
: find yourself an architect who plans the renovation. You can afford it.
Best regards
Dirk Grafe