It is noted that in your case no relatives are helping.
Nevertheless, here is another reminder about this half-truth:
The helpers must be insured through the BG
In principle that is correct, but there is the exception of a favor service and for that the Construction BG does not pay a single cent in the event of an accident.
See the Construction BG website:
Exceptions according to law and jurisprudence
For helpers who work within the framework of a usual favor service in the private sphere, insurance coverage is excluded because such assistance is legally not considered employee-like. To determine whether the activity is more attributable to the private sphere, the relationship between the builder and the helper as well as the extent of the activity must be examined. If the builder’s father briefly helps unload building materials, this will be considered a usual favor in the family sphere, whereas the multi-day help of a friend with roofing work is usually an insured activity.
And here is another example from the Construction Code leaflet
3. The father of the builder, who lives in the neighboring house and with whom there is good and regular contact, has an accident while cleaning up, which involves a small overall time commitment. Due to the family bond, this is to be regarded as an uninsured favor service
For construction helpers who are not legally insured, we recommend considering taking out a private accident insurance.
So the question arises whether help, for example, with painting from the father would be a favor service or not? When preparing a rental apartment no one would think about paying contributions to the Construction BG either.
Those interested can search for
Frontal 21 from 05/11/2010, where there is an article by a lawyer about this.