I think we would actually say "if extra cleaning is required" or something like that.

Definitely not "soiled."

Nappies can be soiled, or a reputation, or very dirty clothes but not an apartment.

I'd beg to differ: apartments can certainly also be soiled.

I would still say "extra cleaning required" instead of accusing tenants of "soiling" the flat which, to me, conjures up images of excrement on the walls as well as vomit all over the kitchen floor.

Of course, but that doesn't mean that apartments can't be soiled. That's all I wanted to point out. :-)

Agree with #6 – another option might be 'there will be an additional cleaning charge if the flat is left excessively dirty'

For the record, I also find "soiled" conjures up images of smeared excrement.

Even though, also for the record, if I found a place in that condition I would _not_ say, "This apartment is/has been soiled." Language is a funny thing, eh?

'soiled' könnte im Deutschen 'eingeschmutzt' heißen.

Aber wie gesagt - hier passt hier nicht.

Den Satz von TDV_0 in #7 finde ich gut und elegant für diese Anfrage.

To me, "soiled" is mostly just the word used to say someone has defecated in their pants, especially if it's not a baby (who is basically expected to do so fairly regularly) when you don't want to use more explicit terms.

Examples: "he soiled himself" or "he soiled his pants."

I think #1 is fine, and #7 is OK, too.