Land price - Buildable or not? Experiences?

Access to the rear part of my parents' property would basically also be possible via my parents' part or through a right of way of the other neighbors. Indeed, a somewhat tricky situation: the neighbor does not need his allotment garden and wants to sell it. The plot might basically be buildable (not yet clarified), but no one wants to build on it because it is just so small. It couldn't be sold anyway, except as it is now, as an allotment garden. For us, in combination with my parents' property, it would provide a comfortable connection to the road, which would otherwise have to be created in another way. The initial idea was probably 400 euros per square meter, but we won't pay that. I can gladly try to get a photo via Google Earth.

I would also like to see the site plan. My property (also next to the parental home) is only 260 sqm, and yet it was buildable...

That will be an expensive driveway. Along with demolition, subdivision costs, and long access routes. You might as well buy a decent plot of land instead of putting on that circus.

I can't say whether the neighboring plot is building land.

From what you have said here, it is building land. Of course you doubt that since you only want to pay for garden land (Ok, ok, 100€ is more than garden land). Here with us, such a plot would be snapped up – more than 18m of street frontage! So don't sweeten the world for yourself, but make the neighbor an offer or plan the access via the parents' property.

Perhaps it would be wiser to buy and develop the 390sqm plot, using the hinterland together as a garden.

Standard land value 250€, neighbor wants 400€, you want to offer 100€. You already realize that this won't work in the current seller's market?!

EDIT: Even on 390sqm you can definitely build a small single-family house with 120-130sqm, that’s 65sqm per floor, so minus 90sqm driveway + carport, there are still 235sqm left for garden, paths & front yard.

With this approach, you may plan without the neighbor's property.