Is the financing plan realistic?

Or things you really didn't think of, for example, the issue of [Verbau] was briefly acute for us, which would have been a five-figure amount. Luckily, that didn't happen.

the situation in Pankow is more than borderline.

Is it about the new development of Blankenburger- / Grumbkow- / Wackenbergstraße?

Exactly, it's about this construction project. The proximity to the main road and the industrial area does not please my partner with regard to small children in a few years. The street was relatively loud and busy. You could also hear airplanes, but they are supposed to disappear eventually.

Spontaneous additional features during construction + outdoor facilities, which are almost always more expensive than planned.

I hope I may interrupt here once again.

I can easily believe that it always becomes more expensive than planned. But since that is precisely unplanned, I wonder how I can determine the right amount of buffer. Is the buffer common practice and does the bank always approve 10% on top for safety, or how is this sold?

Or do they have certain rates they apply for things like floors, tiles, painting, paving, etc.?

Exactly, it’s about this construction project. My partner doesn’t like the proximity to the main road and the industrial area with regard to small children in a few years. The street was relatively noisy and busy. You could also hear airplanes, but those should disappear eventually.

I would perhaps like to add the following considerations. If you want to live in Berlin, there will hardly be any opportunities for "absolute silence," or only for a lot of money. After all, it’s a metropolis of millions; country living is only available to a limited extent.
If you build there in the rear part toward Wackenbergstraße, it is in my view quiet; the Blankenburger Str. would not be my choice either.
The industrial area is mostly occupied by small businesses, the rear part toward Buchholzer Straße (former Bornholmer border troops facility) is certainly intended for residential construction at some point. However, a sales attempt already failed, if I remember correctly.
Well, and the airport will indeed eventually close, perhaps even before the kids are grown.

Negatively, but certainly the parking situation (as always in such developments), here especially due to the adjacent streets (only Wackenbergstraße and Street 41 remain). I find the price somewhat steep, but I cannot objectively assess the consideration (house).
Positively for me is the location (actually only in the rear part), especially with children. Daycares and primary schools (E-Christinen plot, plot at Hasengrund) nearby, secondary schools as well (M.-Delbrück-Gymnasium or ISS Konrad Duden).
Public transport is good (Bus 150, 250, Tram 50 - 10 min to U2 -, S2, S8, S9)
Green spaces (Brosepark, Schlosspark and Karpfenteiche) and a steadily growing family offer due to the influx situation. Supplies also easily accessible on foot.

Is it common practice with the buffer and does the bank always approve 10% on top for safety, or how is that sold?

As far as I know: You have to deduct the buffer from your equity beforehand. So
+ Production costs according to a serious cost estimate or developer contract
+ Additional costs (notary, property transfer tax, realtor)
= subtotal
- equity that you want/can contribute
= capital requirement

The bank decides yes or no and states the interest rate (which depends, among other things, on the ratio of capital requirement to production costs).

Where is the buffer? Without the bank, just for yourself, you do the following calculation:
+ subtotal from above
+ everything else (kitchen, lamps, moving truck, KfW fees, expert, expenses, construction period interest, this and that, something else, insurance, ...)
= total capital requirement for the builder without contingencies (UV)
+ contingencies (UV) (for example, a dinosaur skeleton is found during groundworks. Or a glacial erratic. The only available electrician becomes incurably ill. The developer goes bankrupt. Or something really unforeseen happens.)
= total capital requirement including contingencies

How many euros you put in contingencies depends. On the contract situation, on the circumstances.

Whether the bank gives you that depends on whether the "capital requirement" is greater or less than "production costs." And on your creditworthiness.