Is building a house feasible? We are totally uncertain...

You can immediately add another €20,000 for a double garage to your planning, and photovoltaic... well, I think €350,000 won't be enough — not with the provider you have chosen. They are priced in the same league as our provider.

so I have an offer for a double garage for

W: 5.85 x D: 7.00 x H: 2.45m € 8,600 € 1,135 ...smaller ones even started at around ~8.5k

How much equity is used is also individual. 10% may not seem like much of the construction cost, but can make up 0.5% less in interest.

No loan after 40 years is, in my opinion, nonsense, perhaps no term over 30 years.

EL also costs ... material expenses and labor effort should not be underestimated.

If the factor time does not matter, the question is whether solid construction is not cheaper?

You mean to build up the shell yourself? Well, probably yes.

If you have it built, it will be massively cheaper than an equivalent prefabricated house.

The question is how much equity should be saved? The 10% doesn’t make much of a difference, does it? And if we save for another 3-4 years, we’ll be nearing 40 and might no longer get the loan approved because of our age (I read that somewhere once).

Regarding own work, we would definitely trust ourselves to do flooring and plastering.

The age factor depends on the equity. I feel you are calculating too sharply based on what is currently feasible. If you want children, it will be tricky. Kids are expensive – and the income (usually the woman’s) decreases dramatically!

Also, for that amount, a 10-year interest rate is high risk. The house could be gone again quickly after ten years. Own work is good. But don’t overestimate this item! Especially since the materials for this part still have to be paid for.
There’s still a lot missing in the bathroom section. Also a reason why we no longer consider this provider.
How about a smaller house?

For a smaller house, we would be interested, but then the question arises to what extent that is compatible with our wishes. Since we do not want to build a basement, we definitely need some space somewhere to store various things. Furthermore, I absolutely need an office, as I occasionally work from home. If twins come at some point, we would be in trouble anyway if we only plan for one additional room...