Fill the plot or wait?

Basically, you are right, but local GUs/BUs often have good knowledge of where which soil can be expected and plan certain additional foundation costs, etc., so you can co-finance them and have a buffer if you don't need them. On the other hand, there are also companies that work with external insurance companies so that certain amounts beyond the additional foundation costs would be covered by the insurance, for example if suddenly a boulder appears in the middle of the construction pit and special equipment is needed to remove it, or similar. By the way, you wouldn't be safe from something like that even with a soil investigation now, because only samples are taken there.

First the soil survey, then the planning of the house. Everything else is simply nonsense, as the soil survey can have a significant impact on the planning and thus on the costs. The building ground is always the risk of the builder, which means higher costs always remain with the builder. A soil survey can lead to building with a basement even though it was not previously considered, because significant soil excavation is required due to the underlying soil, or it can lead to foregoing a basement because, for example, the groundwater level is very high. And one should never rely on the neighbor's soil, that can very quickly become a problem.

A big fuss is being made about these expert reports! For centuries, even millennia, humanity built without them, constructing quite large and yet durable houses, castles, palaces, cathedrals, stadiums, etc., sometimes on difficult terrain. And today people are scared to death to put an Allkauf house prefabricated house out in the middle of nowhere without a soil survey. Something has gone off the rails there.

Well, acquaintances got into trouble. They bought a plot of land without an appraisal, planned everything, etc. Then, during the appraisal, clay soil was found on the slope property (slight slope, actually not a big deal). Now the house is a meter higher than planned plus €20,000 in extra costs (actually another €8,000 more, but they did some of the work themselves or left some earthworks undone). A basement would have been smarter, but then they would have needed a new building permit, and with the first permit plus delays with the general contractor, they already lost almost a year. The missing appraisal caused a huge domino effect. Therefore, I would definitely have it done beforehand. In the worst case, quite a lot depends on it.

But between "before I even contacted some BUs" and "after building permit approval," several reasonable points in time pass. My top 3 for soil that is not obviously problematic: 1. Before purchasing the property (then you know what you have) 2. After deciding on a BU, but before signing the contract (there are usually negotiation downtime phases as offers are revised, and for the bank you already have concrete figures. And in this phase, it can usually be easily agreed that it will be credited. And in case of soil surprises, replanning can also still happen) 3. Shortly before the approval planning, definitely before the building application. I still maintain that now it is highly pointless. Or if it is so important to you, just do it. Maybe someone will credit it to you after all.

You need a frost skirt.

Not if, for example, you do an under-slab insulation and then a slightly thicker smooth slab on top. So we are building without strip foundations/frost skirts. It was a recommendation in the soil survey report, or both options were rated equally. The ground floor is already done.