When to start building?

I also think April, because then the chance is greater that it won't rain too often, so the shell construction can dry out over the summer, which is very important to prevent mold from forming.

Why do you only have 2 appointments with the start? I would schedule an appointment in between. Because: -End of June can already be too late depending on circumstances, if complications arise, you are still working on the construction in autumn or winter. -I wouldn't choose April either, because you start first with earthworks/foundation slab and the base walls, and as everyone basically knows, April does what it wants and you still have frost and snow every April, which makes earthworks more difficult, makes concrete harden worse or even has to be mixed with frost protection, and if you don't supervise your construction yourself, I've seen myself how they built a base wall with a gas burner during frost.