I am adding an extension to my existing 1935 house. The problem is that the contractor has done a pretty shoddy job. As a result the subfloor of the extension which is next to the kitchen is a good 3/4 inch higher that that of the adjoining kitchen sub-floor

In another place, where the corridor joins the extension, the subfloor of the extension is about 1/2 inch lower than the subfloor of the corridor.

Is there any way to fix this? At the least the connecting areas should not look terrible...anything to solve or mitigate the problem will be extremely helpful

Thanks

The first thing that comes to mind is sheeting out the addition with 1/2" and the kitchen with 3/4 & 1/2 . But that's a lot of underlayment, and I would venture to guess most likely there wont be that kind of clearance in the kitchen. Especially when adding the additional height of the hardwood.
Other than that, one solution is to mill up thresholds matching the hardwood at those 2 locations. Or you can sheet out the addition with 1/2" and install a threshold at the kitchen. Without seeing the layout, it's hard to tell.
Also , are there doorways at the corridor, and or, kitchen? Or is the floorplan more open? It is common in older homes to see thresholds in doorways.
OR , You can install reducers at those locations, (narrower than thresholds), and would not catch the eye as much.
You would also want to take careful consideration of the direction the wood is running if you go with thresholds or reducers.

Thank you!

I have uploaded a diagram to illustrate the problem. I am really concerned about the Kitchen-Laundry Room (3/4 inch) difference.

I would consider these options....

At the service area , install the tile over 1/4" hardibacker board. This would keep the finished tile height to it's minimum. Depending on the thickness of the tile ( 3/8"-1/2" ), and thinset, and figuring on 3/4" hardwood, that would put you at a finished height difference of approx. 3/4".
If it was my home, I would go with a threshold matching the wood , spanning the width of the casing. I would rabit it out so the lip overlaps the tile. I think this would look good, especially with the era of the home. (There's issues with a reducer at that transition, I wont get into ).

I'm assuming the bath is getting Hardwood??( It doesnt say "to be tiled" ). If that's the case, I would sheet out the bath and small hall addition with 1/2" ACX plywood, problem solved. If the bath is getting tile, I have another plan .

Thanks a ton. Your solutions have been most helpful.