I have Sims 3, loads of expansion and stuff packs, and lots of mods which generally work really well, but one issue I've ALWAYS had - before even installing mods etc - is difficulty in placing fireplaces. They only seem to work in the middle of the room, which I find ugly and inconvenient. I'd love to have a fireplace up against the wall, with the mantel inside the home and the chimney outside the home, but I'm always told that the fireplace "can't be placed through a wall", and that the chimney has to go INSIDE the building as well.

Other houses that already exist don't have this issue, and I can't imagine why it should be a problem.

Do other people have this problem, and is there a patch, mod or solution for it?

Any suggestions are appreciated!

Are you trying to place them on an outside wall, in a house that's built on a foundation? If so, try adding however many foundation tiles the fireplace is, on the outside of the house.
Or maybe the ground is uneven?

Outside walls, inside walls, with foundation, without... it doesn't matter. Each time it tells me the message that fireplaces cannot be placed through a wall. Uneven ground isn't the issue, but I haven't tried adding floor tiles underneath the chimney, which is what I thought you were suggesting at first, and will try anyway!

The only way I can place a fireplace that has a chimney is if the chimney is also INSIDE the room, which is annoying.
I can't imagine why this should be, but I've reinstalled the game on two different computers and had the same issue, so I'm surprised that I haven't read from more people having this issue.