Just before signing a contract with a construction company, what should be considered?

The last 4 days taken:

300 starts to the west
66 starts to the east

Of these, 5 take this route to the south; but all of these 5 after 6 o'clock in the morning.

The last 4 days taken:

300 departures to the west
66 departures to the east

Of these, 5 take this route to the south; but all of these 5 after 6 a.m.

You broke that down nicely right away. Taken together with your information above, it doesn’t sound like a dealbreaker for the property. I have some departures from Nuremberg at about 2500-3000m altitude near me, and they don’t bother me at all. At most, I just glance up out of curiosity when I’m in the garden.

PS: Are you a planespotter?

I have some departures from Nuremberg at about 2500-3000m altitude

But we are talking about 8200-9800 ft.

In the district of Leipzig, the flights when heading south to/over Lindental are at altitudes between 1900ft - 4000ft.

That is only 590m - 1210m.... And sound decreases with the square of the distance.

I checked for fun at my place. In the last 7 days there were 4 planes at 1900m. I noticed 2 of them.

PS: Are you a planespotter?

Unfortunately, far too rarely. But someone with a PPL(A) sits less than 2m from me.

Does it absolutely have to be in the city? The Leipzig surrounding area is very well connected, and usually you can still get plots from the municipalities without developers at moderate prices. Otherwise, I can gladly recommend someone with expertise who can take a look at the construction specification and the contract.

It's always such a pro & con decision when you consider where to build, inside or outside the city. For us, the advantages of the city were clearly more significant, and in Leipzig you can still afford that with 500-600k. Besides, hopefully it's a kind of investment :)
And the plots outside the city don't cost much less if they are well connected to the city by S-Bahn. Only the infrastructure is usually significantly worse then, and the possibility to cycle to the city center (to work) no longer exists (I see this with a distance over 10km).
Any real estate agent would also confirm that a house in the city costs significantly more than the same house out in the sticks.

Regarding the price of the chimney: We paid around 5000 for the chimney (flat rate length for 2-story). However, this was in 2013 without a floor hatch or pressure monitor. The latter should cost around 700 with the electrician. Therefore, I can well imagine the all-in price of 9000 nowadays.

Hm, okay, thanks for the info, overall I find the new build price for a BF relatively fair. Only the sugarcoating of the lifting system and the attempt to sell us the heat pump, which no one knows and costs 19k more than a gas-solar system and consumes about 800€-900€ annually in electricity... that bothered me the most. I have meanwhile sought professional advice, which unfortunately also confirms that these concerns are not unfounded...