Then you just don’t go on vacation for 3-5 years and spend the holidays at the construction site.
With 4000 net, you can still manage after deducting the installment.
As has already been mentioned here, the woman has to study full-time for at least another 3 years. Then the therapeutic training, another 3 years – with costs of 10,000 - 20,000€ (this might be different by then).
Earliest income would be expected after that. In between, there is another child. So 2 more years of parental leave will be added and there is no experience as to how well one can handle full-time study alongside a baby. Realistically, it may therefore take 7-8 years before earning money in the target profession. Or she drops out and returns to the old job.
So it remains a single-income matter. Are those real net income figures, or does the car for you and the children still have to be paid from that? Job security is of course very good and the equity from selling an apartment certainly too. Whether one wants to become a landlord under this constellation and the current political situation to somehow keep the condo must be well considered. If you can get 1300m² in the countryside just 30km away for a cheap 100,000€, the housing shortage probably can’t be endless. We recently had a case here where someone in the region towards the Harz initially didn’t get a loan for their house construction because the amounts for it were not representable in the system for the area. Does such a risk also apply?
Otherwise, the sizing is unclear. With only 4000€, it is significant whether the bare installment is 1200€, 1300€, or 1400€. And why should anyone be interested in a 500m² towel with such low prices for the land? The idea that you would have liquid cash reserves there that you can simply split off and cash in if needed is, in my opinion, bold. In a boom region, okay – but there you pay completely different prices. For the house size: should it now be 160m² or 140m²? That’s surely a good 40,000 - 50,000€ price difference. That’s where I see your biggest weakness – what exactly is it supposed to be? You will hardly get below 2000€ per m² currently, so the project calculation at (160m² * 2000€/m² + 110,000€ land) * 1.2 [additional construction costs] is probably a reasonable rough estimate for a basic model / subdivision house – with an upward tendency. So 500,000€ - 150,000€ from the apartment sale results in roughly 350,000€ loan amount. With a household income of 4000€, this should be manageable. At 2200€/m² you already end up at 400,000€ loan – and slowly approach the maximum loan limit. In the past (rough estimate) that was a maximum of about 110 times household net income.