Realistic building financing? Your opinions

It is supposed to become a utility basement, possibly with a granny flat.

Here you really need to be clear.
As far as I know, with a granny flat it becomes a residential basement. For example, this is higher than a utility basement (but I probably don't need to tell you that ;)). No idea what other differences there are (probably in terms of finishing, electrical, sanitary, etc.).
At least this height would then have to be maintained for the entire basement – even if you only want to use part of it as a residential basement.

The previous scribblers have already written a lot about construction costs. I can fully agree with that. Where do you actually want to build (or did I overlook that)? BW is large.

It should be a mid to upper standard.

What does upper standard mean to you? That of course increases the costs.

I seem to recall that a residential basement would cost around 30,000 EUR

I think that would be more appropriate to calculate for a utility basement. A residential basement (and it would have to be one with a granny flat) is certainly more expensive. (But I’m also just a layperson.)

Overall, I think it will be difficult to keep the 350,000 € with your numbers; with a basement, I consider it unrealistic.

Regarding the financing, I have to say that (if the numbers actually add up that way) a 300,000 € loan with your income is quite a hefty amount (regardless of what your income/expense situation looks like; you haven’t written anything about that). But how do you plan to manage when one or more children arrive and income decreases for at least some time while expenses increase?

Land will cost around €70,000.

That is -- depending on the point of view -- rather cheap, so the granny flat might be better accommodated on the ground floor. Maybe a bit more space is better than the basement. That could be something to check ... and do a bit of calculating. Of course, it depends on the circumstances.