I would say this is the responsibility of the contractor. Invite contractor in so you can get this resolved before occupying the space. If this is DIY, then enlist the help from a contractor. The floor slant might have underlying issues with the joists.

What you cannot have is a brand new house, or a brand new floor, joined to an old one, that doesn't show that it originated in an old wonky house. If it isn't a tripping hazard, it's perfectly fine.

Additions that do not have doorway and thresholds are automatically so much more of a bigger problem that requires a much bigger budget than ones that have a separation doorway. What kind of discussion did you have about designing in such a problem? And the money needed to jack everything up and redo the structural?

Old homes call this "character". And they all have it in droves unless you fully rebuild them.

Pretty simple. Remove the new addition flooring. Level the subfloor to match the existing old flooring level once the removed flooring is put back down.

"Pretty simple. Remove the new addition flooring. Level the subfloor to match the existing old flooring level once the removed flooring is put back down."

My experience with trying that, admittedly in the days before laser levels, didn't work out so well. It was a lot of shimming and sistering with less than stellar results-- the floor in question had kind of a wave in it, not a simple slope that complicated things. I'm reasonably fearless about trying that sort of thing, but I wouldn't have called it simple. Then when you're dealing with slopes of a couple inches things like differences in the height of the window frames off the floor start to become evident.

A laser would have helped a lot!

When the cause of the sag was a rotten sill that needed to be replaced in any event jacking the building up was pretty straightforward.

What has the builder said about the unlevel floors? How far out of are they? How long is that level, get an 8' level at least.

The new floors need to be ramped up to the old since it is lower. Not as simple, subfloor work involves tons of work. Work may need to be done on both sides, old and new.You don't know what type of plumbing, electrical or HVAC that's routed through that area that can create a challenge. You are also, dealing with possible foundation material that you will need to work around, like bricks and concrete. If, you're lucky , you're just power planing the old side down.

As mentioned, this should have beeen discussed at the planning and budgeting stage.

^ that is what our contractor did - he increased the slope of the subfloor across our whole dining room so the old flooring met the new addition’s flooring flush. It is imperceptible since it happens gradually over a large area