Opening up supporting wall on upstairs landing

Hi, hope you're all keeping well.

We're hoping to open up a wall on the upstairs landing of our house, which is a small 1985 terrace cottage. The supporting wall runs up the centre of the house, with the staircase running alongside it, which I think is quite typical of terraces from that era. Unfortunately this leaves a dark internal corridor on one side, which we'd like to open up and replace with a balustrade to let more light in and make it feel less gloomy.

It's a bit complicated as I think this would mean installing an RSJ into the party wall, and with the house being so old, I'm not convinced next door's side would remain damage-free. With this in mind, I was wondering if it's necessary to install an RSJ when you're extending an opening that's already flush with the party wall, as per the pictures attached (it's currently an absolute s***hole).

Also, I know it's almost impossible to price a job without actually seeing it in person, but a ballpark figure would be appreciated if anyone might be able to help with that?

Thanks in advance for your help.

Anybody any ideas?

whats above it in the loft?

look at the loft to see where or if its supporting, thats first step