New floor construction, what would be recommended?

In our old building in the attic hallway, I noticed that the floor is sagging and no longer level. I took a look underneath. The floor is constructed with 20mm styrofoam (now more like ~17mm) and 20mm chipboard. The chipboards were apparently partially installed without grooves, which is why they have pressed into the styrofoam over the years. There is a carpet directly on top. The question now is how to get the whole thing straight again. I assume that the old construction must be removed first as it is unusable. However, the new construction should be roughly the same height as the old one, so about 40mm. I was thinking maybe 20mm styrofoam with 18 or 22mm OSB boards (of course with grooves). Does that make sense? Would 18mm OSB be sufficient or should it definitely be 22mm? Or am I completely wrong and is there a better option?

Even for stability, 22mm with tongue and groove. Check with the building materials supplier about dry screed. The boards also have tongue and groove and the insulation directly underneath (so everything in one board). They are screwed and glued.

Hello questioner. An answer to your question can actually only be given in a satisfactory manner if one knows the overall structure and the connections. You had described them well, but there are aspects which we do not know here in the forum. The old Styrofoam, for example, was probably a makeshift solution. For example, to (through the Styrofoam inadmissibly) compensate for heights of the substrate. As impact sound insulation, Styrofoam is arguably unsuitable for chipboard. The new structure depends on the old substrate. Probably a height compensation is required in several places, which should be done by a dry screed. On this a separating layer, for example made of loosely laid MDF boards, and on this again gypsum fiber, OSB or chipboard panels of sufficient thickness, tongue and groove glued. Depending on use (high surface loads or jumping persons) it may also be necessary to lay the top layer twice, offset by 90° and glued together. These are all information that we do not have. But from the basic structure you should now see how it could "continue". Good luck and enjoyment with the extension: KlaRa

why thermal insulation inside the house? What kind of ceiling is that? If wooden beam ceiling, the thickness of the OSB board depends on the beam spacing. Then [Styrodur] or something similarly pressure-resistant

@ "Grundbuch": At no point in the entire post was thermal insulation mentioned. So why this reference now? Furthermore: if Styrofoam was chosen as a base for the old wood chipboards, then it is certainly not a (former) installation on joists. Therefore, spacing between supports does not need to be considered. It is also contradictory when it was stated "why thermal insulation inside the house?", a legitimate question, and then ultimately "Styrodur or something similarly pressure-resistant" was recommended. Styrodur is a thermal insulation. And thermal insulation is not needed between ceiling floors. And "something pressure-resistant" is again not suitable as height compensation. Replies should therefore contain a justified, logically consistent conclusion between the structural conditions and the corresponding requirements. Otherwise, they are not usable for the questioner who is seeking help in this forum. Regards: KlaRa

What kind of surface should the floor have? You want the same level as before - so the new surface has to be taken into account - if it is not going to remain carpet.