I don't agree with the interpretation in the BRE document, particularly the requirement to include all basement areas if there is one habitable room in the basement. This would imho greatly depend on where exactly the thermal envelope sits.
Die Grundfläche ist nach den lichten Maßen zwischen den Bauteilen zu ermitteln;
dabei ist von der Vorderkante der Bekleidung der Bauteile auszugehen. Bei fehlenden
begrenzenden Bauteilen ist der bauliche Abschluss zu Grunde zu legen.
This is quite a hard nut to crack, even for a native German speaker, as the language used is not actually German, but Bureaucrats, and from experience, not many people agree on a workable translation of Bureaucrats (which entertains the courts ad infinitum). As I read it: the area used is the clear width between building elements, whereby the interior edge of the internal liner is taken as reference point. If no internal liner (plaster, plasterboard) is applied, the wall as such is the reference point.
Yet, this translation is conflicting with the passipedia entry. However, my gut feeling is that something in passipedia got lost in translation, and historical context. In previous German regulations, the floor area was calculated using clear width between walls, and then factoring in a blanket 3% "Putzabzug" = deduction for plaster.
This methodology was eliminated in the new regulation, where the actual, usable space has to be calculated. I believe this legacy issue is what caused the passipedia entry to require unfinished clear width, as they wanted to avoid the blanket deduction. Otherwise, I don't see a logic behind requesting the unfinished surface as reference point, and one really has to ask how "unfinished" is defined eg in the case of an installation layer, or -as the Austrians like to employ - a 6cm timber board in front f the structural element. Logically, the finished interior surfaces should be used as reference points. BRE is picking up on the -outdated - 3% rule, but alert to it not being deducted from the finished area - which is entirely nonsensical.
Further to this: the passipedia entry refers to the definition of TFA as introduced with the CEPHEUS projects. There, however, auxiliary rooms within the thermal envelope are considered with 50% of their area (CEPHEUS-Projektinformation Nr. 35); this is repeated in CEPHEUS-Projectinformation No. 36 (in English), and what I am used to work with. Furthermore, it reads on page 120:
"3.1 Generally, the floor area of a room is calculated from the inner dimensions of the
finished building. For simplification, the dimensions of the building shell may be
used; in this case, for walls with plaster 15 mm of plaster have to be taken into
3.2 The dimensions of the finished building are the clear dimensions between walls
without taking into account wall coatings, base boards, scrub boards, ovens,
If passipedia refers to this only to then strikingly contradict it, I am non the wiser.
As the agreement on the calculation procedure for a reference area matters hugely, some clarification is needed, indeed.