Basement construction

How does it actually work with the basement if I decide to build a house, for example with a prefab house provider? Often you can also book a basement with this provider, then it will probably be planned completely with the house. But actually, this business area is outsourced anyway. However, if I now want to have the basement built separately (e.g. for cost reasons), how does the planning and construction work then, for example? Does the planning still be done by the prefab house provider to correctly place connections, stairs, windows, etc.? Or does the basement construction company take over completely? Thank you

We commissioned the basement ourselves because it was cheaper.
Planning: The house builder provided the structural engineer's requirements and a ceiling plan (openings for electrical / water / stairs).
Interior finishing of the basement + basement stairs + screed in the basement, etc. we had to arrange all by ourselves. The house builder then specified where the chimney would go and where the house connections would be installed (or rather, we had informed the house builder / basement builder of this in advance).
At first, it looked like a huge saving – in the end, it was maybe €5000 but with a lot of stress and problems due to the different contractors and the additional work we had to organize ourselves,....

We did buy our basement from the house provider, but they source their basements from one of the largest basement builders in Germany. Since they do not manufacture the basement themselves, we had a separate appointment with the basement builder (who had already received all the plans from the house building company) to discuss the details such as connections, etc. However, we had already done the preliminary planning of the basement (where which wall, door opening, and stairs) in advance with the architect of the house building company.

And what are the experiences when the basement was not purchased through the house provider?

We also ordered our basement through the prefabricated house manufacturer. However, it was then built by a large basement construction company from Schramberg.

It was important to us to have only one contact person and not be passed back and forth between two companies in the event of damage.

As we learned during the basement construction, there are also different quality standards underlying basement construction.

Our house manufacturer generally orders the highest quality level, which may already offset the additional cost compared to private commissioning.

May I ask a basement question here? We are considering building a raised basement about 50 cm above ground level. Does anyone know the costs compared to a normal basement? We need a white or black tank anyway because of groundwater at a depth of 5m... that means with a raised basement we don’t save on a WW.