Sufficiently deep niche. I can move a less deep cabinet forward if I want it flush.
I also don’t know what kind of carpenters you all have, ours were somewhat more expensive than Ikea, but clearly below the well-known premium brands (well, we don’t have solid wood installations). I would say: have it done by a carpenter, then you get exactly what you want for not much more money. The prices I read here make me dizzy. We were probably lucky.
Key storage: perfect place for a bunch of odds and ends that will always accumulate. I recommend a drawer for something like that, at least then you don’t see the chaos.
