Plot of land 371 sqm, are the expectations realistic?

It is always a matter of weighing pros and cons. A close colleague of mine bought in a suburb of KW three years ago. He has a good 1000 m² of pure garden and a huge house with a granny flat. It takes him 45 minutes. If the train runs and he catches all the connections. However, the train only runs once an hour. He can switch to the S-Bahn, then the trip already takes an hour. If both fail (not uncommon in Berlin), he has to drive by car (up to 90 minutes in rush hour). By bike is unrealistic.

I have long struggled with whether I should buy in Berlin at twice the prices (and more). But I have S- and U-Bahn connections, can still ride a bike (18 km) and then in a patchwork family other factors also play a role (proximity to school/child’s father).

Yes, everyone is advised to make a priority list for themselves, take all circumstances into account and think it over.

If the place in the profile of the OP is the location of the property, I would reconsider that. In my opinion, properties between 500 and 800 m² should still be reasonably affordable there. There are also still some existing properties here that are not as outrageously overpriced as within Berlin.

Yes, it is all a matter of trade-offs. If you live in Berlin, it is not uncommon for you to spend at least 1 hour traveling to work by public transport. Not everyone travels directly from a to b, but rather takes a big detour via c.

I am of the opinion that you can always find at least one very good solution in the countryside. Especially since the regional train from Beelitz-Heilstätten is supposed to run every 30 minutes from next year. And several thousand new apartments/houses are to be built in Beelitz-Heilstätten alone.

I also believe that location is generally more important than the size of the property. What use is a bigger garden if you live in a small village without infrastructure and everything can only be reached by car? That's all time passing by that you can't spend in your garden. That's why we also decided on a tiny building plot (225 sqm, single-family house!!), which is, however, in a great new development area planned with lots of green spaces, with a restaurant, Edeka, butcher, and bus connection practically right on the doorstep. It's worth noting that I can cycle to work and so we only need one car in the household.

Weekly shopping on the way home from work. With the car, you can get everything at once. From a slice of salami to crates of drinks. It really saves a lot of money if you can't buy everything on the go. And the "to go" stuff is insanely expensive compared to a thermos cup and lunch box. I never would have thought so. From home to daycare to work, I have a 15-minute driving time, one village in between, no traffic jams, nothing. Occasionally a deer, hare, fox, or cow on the road. The next supermarket is a 500 m detour. If the location is good, then only the small one works. The rest has to fit. You can't have everything.

Have 426 sqm with the house exterior dimensions 9.70m x 7.7m.

We want a 10Mx7.8M footprint (78m^2 is the maximum) and on that 2 full floors. [...] Of course, a terrace and possibly a garden shed must be added on top

Are you sure that the terrace and the garden shed can come on top? — roughly, for the 78 sqm a site coverage ratio of 0.21 should be allowed (?), as well as accordingly a "II" for the floors, a floor area ratio of about 0.4 and also a suitable eaves height etc.

City dwellers always want to build further away

This has less to do with wanting and probably more to do with being able to (land prices)