I am also missing plenty of essential information to get a clear overview. I interpret the drawings to mean that a porch is to be built onto the first floor of one of the two semi-detached houses to provide the third residential unit (maisonette penthouse) there with the possibility of a "front door." Clarifying would be, not least, information about the ownership structure. The converted new third unit covers both semi-detached houses (this hopefully is a property divided under the [WEG] law; otherwise, is there really an attic spanning two properties?). This will definitely not be a small matter for all parties involved in planning, and I already imagine the financing to be complicated in all conceivable constellations. In addition, the other two parties are referred to as "parents" and "grandparents" (which for part of the couple means the prefix "in-law"). I hope (for the probably already relevant inheritance case in terms of financial security) that the (in-law) grandparents have no other heirs besides the (in-law) parents and/or those willing to convert and expand. Otherwise, the "building plot" in the form of the "split-level floor slab with a descent and approach ladder to the front door" is, in terms of construction dimension, still the comparatively more manageable part of the hassle.
Against this background, the question of whether the 25 cm exterior wall is made of 1980s "aerated concrete" (still requiring some minor insulation upgrades) or 1950s solid brick (eieieieieiei, ouch ouch ouch) almost pales into insignificance.
The lady of my heart also complained again yesterday about my convoluted sentences, but no buddy is perfect ;-)