What loan amount is realistic for house construction?

not feasible with your income.

Only together with the girlfriend. No idea what that has to do with parental support. however, there are exemption limits for property in care, social welfare, etc. That is even good!

That fits roughly if about 1400 is deducted from my 2350 - then I would have to get a loan of almost 1000 euros alone - and my girlfriend is supposed to be left out - once the house is fully paid off, it is her asset and she would have to partly cede it under certain circumstances - we never want to consider/come into that situation.

that’s flawed as well. she’s not supposed to become an owner but supposed to pay nicely?

then I say that’s just dumb for her ^^

+ Land + additional costs + interior finishing (it's basically just electricity, plumbing, heating, screed, plaster, floors + walls, bathrooms...)

Electricity 10 Heating/plumbing with screed 50 Plaster inside + outside (no insulation!) 30 Floor + painter 10

We will get a kitchen and have looked at bathrooms we like for about 8,000 euros.

Then there would still be about 20,000 euros flexibility in "my" financing and the additional 25,000 euros from the building savings contract to be used either for the land or something else.

Please keep in mind that we are not in a hurry and will have almost no labor costs for the finishing work. Exterior landscaping is not relevant for now – that will be done depending on the remaining budget.

if the house were counted, it is her assets and she would then have to partially give it up under certain circumstances

Which circumstances? Owner-occupied real estate assets are exempt assets. There are NO reasons why this must be surrendered. I consider the maintenance issue nonsense in your case, as you have no expertise.

Together with my girlfriend, do you find the project feasible?

So the comparable shell construction costs you €220k + the mentioned costs from you of €100k + land + incidental building costs (€30-50k?).

With your income, you can get a loan of around €200-250k, so you could finance the shell construction without land and everything else. Now ask yourself if that is realistic.