Lighting issue with staircase

XaniMac  I dled the lot and did some testing this afternoon.  Tbh, I was surprised to find that the lot does for me what it is doing for you.  Yet, my test build still doesn't do it at all.  So, it has to be something about that specific lot causing it.   It's definitely some type of lighting/shadow issue imo. Where it is coming from though, I couldn't find.  I did get the shadow to change with some of things I tried, but didn't find a way to make it go away.  Based on one thing I tried; I think it might be caused by something below but still not sure.  One weird thing I found was the shadow didn't change when I deleted the candelabra, nor did the candelabra cast an upwards light.  I did find a couple things though.  When you placed the candelabra, you had MOO on and probably double clicked while placing it as there are two of them, one on top of the other.  And when I deleted the room below, it deleted the foundation and there was a tile that showed a platform edge rather than a foundation edge.  Since you're renovating a premade, it's probably something that the original builder did, given where it was.  Changing it to a foundation made no difference though.  Otherwise, I found nothing out of the ordinary.  

So, the only things I can suggest are either living with it or a complete teardown and rebuild from scratch since I think it's something about the lot from when it was originally built, not anything you did in your renovation.  Save the rooms you've already done and bulldoze the rest.  Tbh, there is no guarantee that it won't happen again in a new build though.  The only place you see it is with the walls up and it wouldn't be seen in normal play.  And you'd probably see it if you took photos in that room.  But with walls in drop down, it isn't visible unless you switch to first person or tab view with the camera down low inside the room.  I'm going to have another go at it tomorrow just to make sure I didn't miss something.  I'll post again if I find something but if you don't hear back from me, it'll mean I didn't.  Sorry, I couldn't give you better news.

Hope this helps.

I tried your suggestions but couldn't seem to get it to work, I've got my lighting settings on very high too

I've uploaded it to the gallery if you wanted to take a further look! I'm surely doing something wrong haha

Thanks 😊

Wow thank you so much for the dedication! I'll try following your steps & will keep you posted, thanks again! 

XaniMac  I did a test build to see if I could reproduce what you are getting.  I tried to match your build as best as I could.  My test build didn't have the shadows above the stairs that you have in either build mode or live mode nor does it show up using the Tab key nor in first person view with candles lit or unlit (See my pic below).  I even tried placing spandrels above the fences around the stairs which made no difference.  So, I am not sure what you have going on exactly.  When I first saw your pics, I thought it might be the difference between how shadows work for candle fixtures and "electric" fixtures, but it doesn't appear to be that.  I simply could not find a way to get the shadow effect you show to happen for me.   So, the only things that I could think of that might be causing it are: 

1. Different game settings. I run my game on Utra with all the other settings on either high or very high.  My lighting setting is very high. Or the color and density setting on the light fixtures.  I left mine on the default. 
2. Or if it could be caused by a lighting mod or possibly if you use other mods that somehow affect lighting levels. 
3. And finally, if you use an add on program such as Gshade or Reshade.  If you are using any mods or add on programs, I suggest you remove them and test to see if the issue is still there. 
4. One last thing that I thought of is it being possibly due to the order you did things.  I built my test build in this order after building the 2 levels of one room each.  I built walls around the stairwell area, changed the walls to fences, deleted the floor, installed the candelabra and other lights and finally placed the stairs.  I configured the turn in the stairs elsewhere on the lot before moving them into final position.  To match the size of the candelabra in your pic, I used the "[" once to make it smaller after placing the stairs. 

In the end, I simply couldn't reproduce what you have.  Maybe something I've posted here will help you find the cause and fix it.  I'm sorry I couldn't be of more help.