Just a question, how do you come up with the 300k€ including additional costs? This amount has to come from somewhere. Is it your limit or the amount the bank told you?
What you have listed as your wishes really clashes with the construction budget. It simply doesn’t fit.
125sqm with a basement costs something different than 150sqm without a basement. Wanting a photovoltaic system but then saving on underfloor heating also doesn’t fit together. Not to mention setting up a DIY store carport.
I suspect that if you want or will carry out your building project with your current knowledge and wishes, you will run out of money faster than you can blink.
But anyway, the question was: is it doable...
1. You will definitely get financing in the current situation where banks want to get rid of their money.
2. But that by no means means you should believe that the financing is healthy.
3. Honestly, 3-4 years ago with this salary the feasibility would have been "extremely tight". In 2019 or 2020 it’s just madness. Five years ago my savings bank told me under the same financial conditions: "You better finance only 250k€, it shouldn’t be more." Only problem is that five years ago you couldn’t get anything for that sum either.
4. Sure, you are 10 years younger than we or most people building here and can pay off longer, but at the cost of higher interest rates, which reduce the repayment again.
5. Many who wrote here have salaries somewhere around 5-6k€ net and have built in a buffer alongside the financing with the same or 100k€ higher financing amount. You have zero buffer.
6. The own contribution you listed is within the usual range here. Except the electrical issue. Here you will probably save about 5k€ compared to individual trades contracting with a standard installation. That’s it.
7. The house will cost you more than 300k€. For your sake, I’ll calculate for 2020 (which is when you will start building due to lack of capacities) with 1800€ per sqm excluding additional costs. Many already budget 2000€ without additional costs for 2019.
At 150sqm without basement that makes 270k€ (and that’s calculated with a wrong value) without kitchen, without landscaping or slope consideration, without carport, without internal development to the house, without administrative costs, without sampling etc.
8. So think again about the whole project. The decision is yours anyway. From my point of view, choosing between house or children is, to put it mildly, nonsense. If the wife/girlfriend “feels” the desire for children, you either get a child in the financially tight situation or you push through your own way, block everything and are single faster than you can look. But you don’t understand that as a man. I don’t understand it either.