Baseboard, pipes and the gap behind a flat back
A case with a flat back and a wall with a baseboard cannot both be right. Something has to give, and it is usually the depth figure you were counting on.
What the trim actually does
Baseboard is usually between half an inch and an inch thick. Push a flat-backed case against it and the back panel meets the trim rather than the wall, so the whole piece stands that much further into the room and tips very slightly forward. On a thirty-inch sideboard nobody notices. On a six-foot bookcase, that fraction of a degree at the bottom becomes a visible lean at the top.
Radiator pipes, cable trunking and a badly finished corner bead all do the same thing, and they do it in one spot rather than along the whole run, which is worse: the case then rests on one point and rocks.
Three ways round it
The neatest is to notch the back panel. Mark where the trim lands, cut a shallow rebate along the bottom edge of the back with a sharp knife and a straightedge, and the case drops back flush. It is a five-minute job on engineered panel and it is invisible once the piece is standing.
The second is to accept the gap and use it. An inch behind a sideboard is a cable run, and it stops a plug being crushed against the wall. Just add that inch to the depth when you check your walkway.
The third, for a tall case, is to shim the front feet by the same amount the trim holds the back out, so the piece stands plumb again. Anything thin and rigid works. Do this before you anchor, not after.
Check it with a level, not with your eye
Put a level on the top surface front to back as well as side to side. A case that reads level side to side and leans backward is fine; one that leans forward wants shimming immediately, because that is the direction furniture falls. Once it stands true, doors that would not line up usually line up on their own.
Pieces this applies to
Shelving stands tall and shallow, which is exactly the shape that shows a gap at the bottom. These are the ones people put against a wall most often.
More from the workroom
Measure the wall first, then open the catalog
The three numbers to write down before you look at a single photograph, and why the smallest of them decides.
Anchoring a tall case so it stays where you put it
Find the stud, use the bracket that came in the box, and understand why drywall alone is not an anchor.
An afternoon on the floor: how flat-pack assembly actually goes
Set up properly, sort the hardware first, and the case that looks like a two-day job goes together before dinner.