Unable to place stairs against interior wall

@MalinKanji 

I tested this and stairs will place with no problem against the exterior wall.  I tried to duplicate what I saw in your pics as closely as I could and had no issues.  And I tried it with both a regular foundation and using a platform as the foundation.  Both worked.

But the thing that I see in your first pic is that where you are trying to place the stair does not have enough room.  In that pic, you are leaving only two tiles for the stair.  Mine wouldn't place when I did that.  Moving over 1 tile, the stairs will place.  The only difference I had was I had a different error message.  Mine was "Can't Intersect with Walls" where yours was "Modular Stairs have no Steps".  

Hope this helps.

Did you try placing it along the middle of the wall (rather than the edge/corner) like in my picture also? I wouldn’t be able to do as you pictured anyway in a proper sims house as the 1st floor wall would be in the way. 

there’s also the fact that yes, in my first picture there would only be 2 tiles at the base of the stairs. But last patch (and as visible on the actual stair markings on both the bottom and top of the stairs when correctly placed) the game recognises they only need 1 square of space both top and bottom to function. 

I can somewhat understand an issue if I was leaving it with only one square at either top or bottom, but I’m not. There’s 2-3 squares of space but apparently I’m unable to place the stairs??

@MalinKanji 

I was able to place the stairs with no problem pretty much everywhere there was room inside the walls for the footprint.  A stair in a room with the low wall requires 5 tiles.  On the floor where you place the stairs, it needs 1 for the lower landing and 3 for the stair object itself, plus 1 tile on the floor above for the upper landing. It only takes up 4 tiles on the lower floor though.  And if look at a stair from the floor above, the opening a straight stair makes is always 3 tiles, regardless of height (low, medium, tall), so you have to have at least 3 tiles for the stair object on the floor below plus one for the lower landing although the taller walls do require 4 and 5 tiles below if you keep the default stair object base.  I usually delete the base so I can use the extra space underneath.

I did try placing the stair pretty much everywhere you can.  Against the perimeter walls, middle of the room, in the corners.  Worked everywhere as long as there was room for the footprint.

The only thing I can think of that may be affecting you is what is above the floor you show that is not shown in the pic.  I tested with nothing above.  If you have a room up top that is the same size as the one below, where you show the stairs would conflict with the perimeter wall above as there is no place for the upper landing.  I was actually surprised while I was testing when the stairs placed without anything for the upper landing to land on since I didn't make any walls for the upper floor.  Sims couldn't use the stair that way though since the upper landing had nothing under it.

One other thought.  What is your base?  Did you use the wall or room tool to make your room so that it has a foundation?  Or did you draw a platform and then put the walls in?  Platforms do cause some odd things to happen.  And if you made a platform and deleted it and then drew your walls, platforms had an issue that you couldn't place anything else in the tiles where the platform had been.  It's been fixed mostly but I still have it happen from time to time in my game.  Exiting the lot to the map and returning usually fixes it when it does happen.  Sometimes, it  requires a save and quit and restart to fix it but one or the other always works for me when it shows up.

Hope this helps.

Sorry for the delay, the account where this map was on is the account I use on my work PC.

The issue still persists even after having the PC (and game) shut down for a couple days. I've compiled a little collage with full layout of the house I'm trying to build along with extra details of the issue.

I do have platforms on the build, but only on 2 of the roof pieces which are independent from the offending area of the build. I made those platform roofs via the 'Flat Square' tool on top of the roof.

To make the house base itself, I used the square room tool to build my base shape. I then kept interior walls in place while I placed decorative trims and different exterior wallpapers. I then deleted all my interior walls (when each of the the floors are selected, the whole floor is highlighted as one piece and not multiple 'mini rooms'). Once walls were deleted, I tried placing my staircase to connect the ground and 1st floor. And here we have the issue.

As you can hopefully see in the images. I have no base foundation to the house (never have) and the only platforms on the property are 2 roof sections in different areas of the house to the area where I want my staircase.

bb.moveobjects has not been reactivated thus far. Although this cheat shouldn't affect where the stairs are placed atm anyway.

@MalinKanji 

Thanks for the extra info.  From your pic, the only thing I can really see that you are trying to do is place stairs on top of each other where they are stacked in a sort of stairwell arrangement.  Correct me if I am wrong, but what I see is the stairs work on the 1st floor and not on the ground floor.  And I can see the reason why too.  The easiest way to get stairs to place in that arrangement is to start at the top and work your way down.  What I see happening in your pic is not due to the stairs being against the exterior wall.  It is due to the stair base of the upper stair blocking the placement of the stairs from the floor below.  The game sees that as a solid object and won't let you install the stairs directly below it.  So, what you need to do is delete the stair base.  Then the game will let you place another stair directly below it on the ground floor.  Of course, that will leave the stairwell open.  You can hide it with a full wall section or use a railing around the opening since we don't have a sloped wall section that works with the slant of the stairs so we can hide the opening.

If you are willing, I will take a look at the build if you upload it to the Gallery and let me know here in this thread just to make sure there is nothing else going on.  

Hope this helps.