Construction of a 600m long water connection line, any experiences?

Are you really about 40m lower? Or was something bigger expected?

I calculated with DN40 myself and arrived at about 0.5 bar pressure drop through the pipe using an online calculator. Regarding the slope, it depends on where the pipe starts; the location lies on a hillside. I am attaching an elevation profile (from Tim-Online), according to which there is about 8 m of slope from the town center (the supply is on the left)

30l/min is already quite a lot, a normal showerhead delivers 12-15l/min.

If I have two showers running simultaneously in the two-family house, does that work?

The costs consist of several factors; initial information for the bare pipe is around €2,500 (internet, without delivery, without installation) or €6,000 (local civil engineer, pipe laid in the utility company's trench). Then connection has to be made at both ends, presumably with a shaft, meter, and pressure reducer. Maybe notary fees will also apply. And worst case other requirements, e.g. deeper installation.

You see. Your information is absolutely plausible. DN40 has a loss of 0.23 bar at 30l/min over 600m. DN25 already has 2.7 bar here.

That is why it is simply not plausible that 50 over 150m would mean such an insane additional effort that would become unaffordable.

Because making the damn pipe 2 sizes bigger. Hardly anyone cares about that. A digger bucket for the trench is 40cm? You can fit DN25 in there and DN40 just the same...

Unfortunately, I am not an expert. I was just given this argument by the supplier. Everything would be feasible. The 25k€ for 49m was just expensive enough for us. Maybe it was also due to the 150m distant feeder to which we would have been connected. Ultimately, I just wanted to share our experiences with the OP.