However, everyone must be able to cover their own financing as well as living expenses. Living expenses as a flat rate of 700-900€ per person. If you cannot even pay off your car from equity, you should think very carefully about building a house. Otherwise, there is also the issue of land register security. If you both want to finance with the same bank, it will - if it is actually that crazy - want mutual guarantees. I would demand that, at any rate.