Cost estimation for bungalow with basement KFW40

A bungalow is generally somewhat more expensive to build than the typical "2.5-story house." The bungalow has a poor ratio of exterior surface area to living space. Therefore, additional effort in terms of insulation (especially on a slope) is to be expected.

Basically correct, however, the OP has simply misused the term bungalow here: what is apparently meant is a lowered two-story house that resembles a bungalow from the mountain side.

It is planned to build a basement due to the hillside location, which is to be done mostly by own effort. Half of the basement is to be used as living space.

We did something similar, although "hillside location" has to be considered individually. From experience, I can tell you that the portion of living space in a basement is just as expensive as the rest of the living areas above. Even the pure basement rooms, e.g. storage room, house technology/connection room, etc., can be somewhat more expensive compared to a purely fully usable basement, because you have to assume the living room height of the living areas (in the basement); please check the building regulations for that.

One more thing: basement on a hillside and then build it yourself? Are you sure?

We are almost finished with our single-family house with KfW55 standard and are running at 3,300€/sqm. That includes incidental construction costs and a double garage. However, exterior work is not included. Own work so far only rudimentary. Especially do not be too optimistic with this point...