Replace individual boards or remove the entire floor?

If you want to tell the story of the house, you could cover / fill the areas without floorboards with a different material. I can imagine that a simple, modern seamless floor with a well-chosen contrast color can create a very individual and great effect. Somewhat elaborate, as it does not fit the current substructure, but from my point of view very appealing.

I have no idea about the subfloor and how to do it. I would refrain from using pieces of "Fremddielen". Just post the floor plan with a marking where the terrazzo ends and the floorboards are. And how the furnishing is intended. If, as it seems, the kitchen is set up with an island there, you can somehow frame it, which contributes to the appearance.

I would actually also (as mentioned by other forum participants) completely remove the old floorboards, thereby also checking the exposed support beams for any damage/loss of load-bearing capacity, remove the old infill between the support beams, and refill the gaps with a modern inorganic material. I do not see the need to consider impact sound aspects, as the room is probably on the ground floor. Moreover, in timber-framed houses, high requirements for sound insulation are difficult to impossible to achieve. You are not doing yourselves any favors by installing a patchwork possibly chosen for reasons of (construction) cost minimization. "Bite the bullet, close your eyes, and get through it" is probably the motto here. Furthermore, once the dismantling is done, you can also check/measure the flatness of the support beams and, if necessary, correct it with little effort. Last but not least, please remember that old nails in old wood do not always have the same friction values as before. This means: due to, for example, drying processes, the old nails can allow movement of the old floorboards, which will cause squeaking or creaking noises with every movement of the boards. Trying to correct this afterwards is almost equivalent to a complete renovation of the floor construction. Conclusion: Better to tear everything down and rebuild it new, then you will definitely be on the safe side for future typical living comfort! ----------------------- Greetings to all: KlaRa