ᐅ Cost Estimate for Strip Foundation of a Bungalow

No, what he means is that you need to excavate much more than 20cm (8 inches). Plan for about 1 meter (3 feet).
At the same time, you can’t use all the excavated soil for backfilling or filling because you need load-bearing material for that.
That means you will end up with around 400 cubic meters (14,125 cubic feet) of soil on the property, and if you’re lucky, you can use 100 to 200 cubic meters (3,531 to 7,062 cubic feet) of it. So, expect to have 200 to 300 cubic meters (7,063 to 10,594 cubic feet) to be disposed of — which is expensive, and even more so if the soil isn’t of very good quality.

I mean that you need to add a significant amount of fill to make a 20x20 meter (65x65 feet) area reasonably level and ensure the house does not end up leaning. Consequently, the excavation contractor will likely present you with an invoice for additional work and gravel.

Of course, you can also model a levee around the garden, but this is usually not permitted...

The magic word for this is "Hügelbeete" (raised mound beds).

I hope you have a construction supervisor/site manager on your side to support you.

Get several quotes so you can tell if the price is right. However, you must (!) define the scope of work clearly to make them comparable.
If you can’t do this yourself, pay someone to do it for you.

Separating drainage and foundations is a typical interface issue... This requires careful coordination regarding levels and where pipes need to be installed in the foundation area.

Okay, now to the core of my question: can anyone estimate the cost of the strip foundation?