Buy now or wait a little longer?

In the location you prefer, I actually don't see a plot + 160 sqm house construction for a budget limit of €800,000. For that, the land costs in the mentioned districts are actually too high. From €1,000,000, it could be feasible there. For the overall project, you would probably have to go further out.

In such locations, it should not be difficult to find partners for a semi-detached house construction.

In the location you prefer, I actually don't see a plot + 160 sqm house for a budget limit of €800,000. The land costs in the mentioned districts are actually too high for that. From €1,000,000 it could be feasible there. For the entire project, you would probably have to go further out.

We are also willing to make compromises on the house. We don't need anything big. Simply "just" 5 rooms + possibly a study. Unfortunately, the location is very important to us. But: we don't need a house at all costs. It would just be nice, nothing more :)

First the plot and later the house construction would also be my preferred option. With your method, however, we would then lack the equity (since our savings rightly go towards repaying the money from our parents) for the house construction. Wouldn't we then look bad to the banks?

No, since the plot represents your equity. You could pay off the 150k within three years (with your mentioned 4000€ savings rate). Then the bank will gladly give you the 500k-600k for your house.

We are also willing to make compromises when it comes to the house. We don't need anything big. Simply "just" 5 rooms + possibly a study. Unfortunately, the location is very important to us.
But: we don't need a house at all costs. It would just be nice, nothing more :)

Okay. But where you absolutely want to build, it isn’t possible within your budget. Even with €1,000,000 it will be very tight. In Panketal it would probably be possible for you. And you would also avoid the school stress in Pankow with three children there. But if it absolutely has to be Pankow for you, then probably not in your own new build.

Or maybe a townhouse or a condominium from the developer after all. That way you're a bit more in control. Have a look at [Pankower Hofgarten], but I don't know if anything is still available there.

When it comes to the house, we are also willing to make compromises. We don’t need anything big. Simply "just" 5 rooms + possibly a study. Unfortunately, the location is very important to us.
But: we don’t need a house at all costs. It would just be nice, nothing more :)

But with 5 rooms plus a study, you’re automatically already looking at something big...
You need 3 children’s rooms at 14m² each, a bedroom at 16m², a bathroom for 5 people and usually that’s on the upper floor. Plus hallway, plus "staircase". So you’re already at 80, 85m² on the upper floor. And if you have 80m² upstairs, it will be difficult to have less on the ground floor. So you end up at 160, 170m².

Well, study on the ground floor. I would recommend that anyway, short distances to the coffee machine, to the door when it rings, Bluetooth reception while doing that.