How two install hardwood floors in a long room with a subfloor that has a plane change in the middle?
Yeah, I would nix the sleeper idea too. Sand away or rent/buy a Power Planer or Belt Sander to take the hump down. Or, remove the subfloor rows (assuming they're running parallel with the hump) &amp; replace them with thinner stock or plywood of the right thickness.

On the flip side, literally, if you have access to the floor structure from below you may find repairs needed there that can bring the old floor up. The old structure may very well be led-in into the bricks will be found to have crushed or sawed-down the bricks or mortar to explain the hump.

The joists would just need to be shimmed up &amp; you may elect to do them all to fix other such deflection issues in the rest of the original house. Be cautious though because fixing the structure can cause cosmetic problems if you find big drops that need actual jacking.