Does my budget fit the construction project?

With your calculation and income situation, you can build and sleep peacefully. In the end, it will surely be another 50K€ more, furnishings including kitchen and basement are tightly budgeted - of course, I don't know what furniture you currently have and whether you are moving a kitchen. But those also seem to be easily manageable.

20K for furniture, kitchen, etc. I find a bit tight. It also depends on what and how much you take with you.

I also think +50K buffer and it fits.

Financing: 400 loan + 200 equity over 20 years at 1.26% interest and 3.7% repayment results in a 1653€ rate and a term of 23.3 years, remaining debt 63k - without [Wohnung], so it fits.

A basement would actually be important to me. Preferably also with a hobby room. That’s the only luxury I would allow myself. Can I get that with 50K in addition to the approx. 2000€ per sqm of living space? There is a slight slope of about 1m along the length of the house (as I would place it). You could then let the basement windows look out of the ground to let some light in. Otherwise, the house is not planned in much detail yet, haven’t been to any company yet. Brick by brick, gable roof, without many frills, practical floor plan, downstairs living area, kitchen, toilet, storage room. Upstairs parents’ bedroom, 2 children’s rooms, bathroom. Ideally somewhere a home office as well. OK, furnishing might be tight, but we already have some things. Is a spacious garage with space for a car and bicycles for 20K realistic? I’d prefer gas, but that’s not available, so it will probably have to be a heat pump, whether we like it or not. We went to our house banks some time ago when we started looking, and they promised us an even higher budget. That was before our son, when we said we planned to have children. It wasn’t as concrete as now, with the first child here. We probably should also go to a broker.

Won't 50K be enough for that?

60-90k are rather realistic

Is a spacious garage with room for a car and bicycles realistic for 20K?

here I would rather go for 40-45k (maybe a prefab garage is cheaper?)

A basement is really a matter of preference. In our current house, we have a basement where no one likes to stay, despite a workshop and a music rehearsal room. Things are stored there and forgotten until they suddenly get in the way. No, we didn't want a basement anymore. A great garden house is ideally suited for a workshop if a) the property is large enough and b) there is a building permit for it.

A prefabricated garage as a double garage in the size of 6x7m is well possible for 25T including foundation.

Regarding the basement, I completely agree with , we ourselves have checked many companies and are now at 75T plus about 25-30T for earthworks (however, slightly larger dimensions, about 9x12m).