Builders ruined a floor :(

[Dumbo]

I would think about more than noggins if it's as bad as the op says I would think about some osb or ply flitch plates

 

[Sean Kelly]

Have you tried bouncing up and down in the middle of your room? That's what I do when I quote for work. How bouncy is the floor? Put a near full glass of water at one end of the room. Does it spill when you bounce? The amount of job I've had to turn down because of 'deflection' (amount of bounce). This can be caused by not enough joists or noggins or poor quality ply or a combination of them all. I even had to say no to tiling my best mates kitchen/diner because of the bounce.