Completely close the ceiling, unused staircase downwards

Hello craftsmen!

As mentioned above, we would like to completely close off an unused staircase that currently leads from our apartment down to the granny flat. Removing the staircase is not the problem so far. But what happens next? The closed-off area should then be walkable and possibly serve as an extension for the current small bathroom. Since we will not be accessing this area, a detailed guide would be beneficial for us to at least get an impression of what awaits us and what we can still do ourselves in this case.
If more information is needed, just let us know.
Otherwise, we are curious about how such a process proceeds and what consequences it entails.

Best regards,
Melanie

What type of ceiling is it, concrete, wooden beams, hollow block ceiling, how thick is the current ceiling?

16cm reinforced concrete, 4cm insulation, 4cm screed.

Glue reinforcement bars into the concrete ceiling, form and concrete the hole from below, close the same floor structure as existing. Or install a wooden beam construction and close from below with gypsum board, insert insulation, OSB board + dry screed. If the apartments are occupied separately, the concrete solution would be better for sound insulation.

Great, thank you very much! The second [Moglichkeut] sounds easier at first!