Construction costs for 120m2 excluding land

The ground is not completely level, a tree and a garden shed will probably have to be removed.

Is this even building land? It reads like garden land. You are not allowed to build on that. And then check what the floor area ratio still leaves available for another house. A 3-meter access path across the depth of the "old" property must also be regulated and maintained later if not additionally paved. Good luck