Backflow protection for greywater

We have "only" a washing machine, dryer, and a utility sink in the basement. The outlet of the wastewater pipes is located approximately 75cm above ground level outside. However, since the basement area with the mentioned wastewater pipe is below street level, a backflow prevention device is necessary. Especially because the building insurance does not pay out in case of damage, such a device should be in place.
We already discussed this topic during contract negotiations and wanted to plan for an electronic backflow prevention device. Then it was said that this is not necessary for grey water, but only if, for example, a toilet is also in the basement. It was said that a simple safeguard via a "ball valve" is sufficient. Now suddenly it is said that we do need an electronic device, which leads to additional costs. We had already raised this issue beforehand and seemingly clarified it.
Question: Is a ball valve (I am not an expert, but it sounds like a purely mechanical solution) sufficient, or is an electronic backflow prevention device mandatory? Is it simply installed into the pipeline in the basement? Costs?

Electric backflow flap costs approx. €2500. For greywater, a standard one is sufficient. Both devices must basically be professionally maintained every six months. I would be interested in how insurance companies handle this in practice.

A normal one is sufficient for greywater.

Does a normal one then cost less? Handling of the insurances: I think in case of damage it will be checked whether one was present?

Available, of course. But maintained as well?

A regular one costs about 700.
Your shell constructor will probably get a discount, but will charge something for the installation.

You can upgrade the flap later. Therefore, it's best to install an empty conduit or cable right now.

What is to be thought of the variant "ball valve" as protection? The keyword has also been mentioned.