Needing to replace rotten wood in my flat

Well, firstly, it would be a lot easier to work with the occupier downstairs! It sounds like they are his ceiling joists as well as being your floor joists! But there may be a way to do it all from above.... Your floor needs to be taken up carefully starting from one edge. Don't take it all up, because it could de-stabilise the whole structure. I would still want to put some props in the flat below if they would allow that. You should then be able to install new joists alongside the old ones but using timber that is just slightly bigger to ensure new floorboards do sit on the new joists and not the old. You could screw the new ones back into the old ones to help support the ceiling below. I would use a timberlok fixing for this. The new joists will need to go through on to the wall plate, or into the wall, which may be the trickiest part as the wall may need to be broken out at each point that the joist goes in. Treat all the old timbers with Cuprinol 5 star, especially on at the ends. Once you have replaced 3 or 4 joists, get some boards screwed back down to support the structure. Use the loft size boards so that you can start replacing without having to take up too much. You may end up with a slight height difference (10-15mm) between floors room to room, but you should be able to deal with this with some levelling compound.

Hi. If the sides of the floor, the edges that are holding the middle joists , are in good condition, you can strength the floor from above. If tou have a good carpenter

Personally, I would not fit new joists or flooring to the existing floor. If it’s rotten and degraded structurally, I would want to find out what has caused the rot in the first place? Unfortunately, I don’t think you can carry out any suitable repairs without the cooperation of your neighbour. You need to show them the extent of the damage and potential danger of not working together to resolve the problem. From your description, doing nothing will only lead to the floor failing which will impact on the property below. If your doing a job you need to do it properly.!