Basement extension beyond the building envelope

I am planning to extend my house by adding a Sutterain apartment - that is, to build a living basement - beneath the terrace or into the garden (that is, basement under the garden) - must the building window also be observed for the basement?

short crisp answer - is there perhaps any reason for this? - Because - even in (from Meier's position of course difficult) objective attempts at consideration - who - except me - suffers damage from this? The terrace is there anyway, whether there is a cellar underneath or not - same goes for the garden ...

short concise answer

It was also a short question with little information.

is there maybe any justification for that?

In Germany, land use is regulated by planning law, which is mainly described in the Building Code (Baugesetzbuch) and the Land Use Ordinance (Baunutzungsverordnung). In this, regarding the buildable area (§23 Baunutzungsverordnung), no distinction is made between above-ground and underground areas.

who - besides me - suffers damage from this?

The soil, because it can no longer fulfill its natural function.

The terrace is there anyway

Since you have so far withheld the development plan from the forum, one would have to carry out a few "if... then" nested conditions, which I will spare us now and simply question the permissibility of the terrace, provided it is outside the building window. I commented on this earlier here:

Terraces are a controversial topic

But even if it were permissible there, it makes a big difference whether there is soil or living space underneath - keywords: lighting, ventilation, escape route, parking space, site occupancy index, floor space index

Note: Regarding my own quote, I would like to point out a feature that was unknown to me for a long time, that the small white triangle links to the complete post.

In the course of the greening of our everyday thinking, I no longer know of any [B Plan] in my immediate vicinity that allows terraces outside the [Baufenster], let alone basement apartments. The purpose of [Baufenster] is also to maintain the distance to the neighboring property, neither to steal the sun there nor to get on their nerves, which naturally limits terraces and basement apartments as well. In the case of slopes, strict [Baufenster] also implicitly limit excessive excavations or embankments. Karsten

In the course of the greening of our everyday thinking, I no longer know of any B plan in my immediate vicinity that permits terraces outside the building window

... this should also concern the border strip, to make it clear once again

Hmm? Border strip? What do you mean?