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Source: https://writers.stackexchange.com/a/43505
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

As others have explained, Chekhov's gun is a bit of a red herring in this context. I think what you want is to be fair to the reader, not to make it seem as though you pulled a convenient basement out of your ass late in the day to get you out of a plot hole.

What I would suggests is to scatter the information about the basement and padlocks throughout the narrative, rather than lumping it all in at one go. In this instance you could break it does into elements such as

- if the house isn't remote, a character might be in a neighbouring house of similar design, which has a basement. It only needs to get mentioned in passing, someone says somehting like, 'excuse me I just need to pop down to the basement to empty the dryer'.
- the padlock could be foreshadowed with comments about the losability of keys, someone gets their bike stolen because they dropped the lock key and Housemate remarks that's why he never relies on keys for anything really important to him.
- The forbidding of access can be established as part of the housemates character not to let people have ready access to his spaces. You could even have him making a point about locking his own room with a key, which might lead an attentive reader to wonder what the housemate _really values_ if he trusts his room to a key-lock.... leading to a satisfied 'Ahhh, I _see_ moment for those readers when he locks the MC in with a combination padlock.

Original score: 5