u/ChiefZeroo

I have two rooms that i want to be connected I realized that the two floors will be about 1cm (.4 inches) different in height. The smaller room is higher and the much much larger room is lower. I could raise the subfloor and the flooring in the large room but moving this higher would make problems in the rest of the house though this has joist easily accessible. I could also lower down joists in the small room but this room I was going to keep the old flooring and make it the subfloor (no current subfloor originally in my old Japanese house). That floor is still attached.
There are boards that are 3cm (1.2inches) thick made for tongue and groove flooring I was going to use . So i was thinking about using the same kind of wood but planks that’s are 2cm thick without tongue and groove though, assuming i can find some.
The other option I thought of was maybe tapering the the planks down 1 cm over 90cm (35.5 inches) of length but this room is made (you can have it open or close via sliding door walls, I would reinstall with very little floor rails.) I would have to do it on the other wall as well to match the hallway but would need to Make the 1cm slope short, 20cm (7.9inches) or across multiple planks.
Normally a small raise wouldn’t be great but I need wheelchair access.
What do you think would be the best move here? One of these or something else?
Please keep in mind that this in a 75 year old place and perfection really isn’t possible without a complete rebuild which I can’t do. I’m ok with functional, looks fine, safe and not (unneedingly) expensive.

So I bought an old house out here in Japan where I have lived for some time. It’s around 75 years old but was built to replace an older house that burnt down so it has the old style of building. One room that was renovated some time later has a floor that is falling apart. The joist seems fine (from above but will check closer later) but the flooring is peeling and weak. I read that this is normal with this kinds of wood from that era and if there is any moisture at all. I will be tearing it out this weekend. I want to raise the floor 10cm (4inches) to match the large portion of the house to make it wheelchair accessible. While I will be doing this in other rooms too, this room is the biggest has has the most extreme raise and is the only room that has bad floors.
My plan is as follows, is this bad?

after tearing out the floor I will fix any joists that are in bad condition and possible add some floor supports as needed. I will add a vapor barrier as well on the ground. I will then add more joists on top of the old joists but perpindicular to them to raise the level up some. Add insulation in there and then put in sub floor and then hardwood flooring. I was wondering if I should put a “sleeper subfloor” over the old joists but not sure if it is worth it or just expensive.

I will also need to change door heights but the doors are rather large so it should be fine. What do you think?