1. Permissible, but perhaps but not wisest decision. Regardless still up to the believer to need to be clear in their own conscience, and to for other believers to bear with them in love. No requirement for church discipline.
The bible is silent on living together before marriage, as cohabitation is a recent some might say peculiar (as as my pastor said its wanting to get all the benefits of marriage whilst still living as single and in the end doing neither at all well) invention of 20th century western society. Living together is no more directly called a sin than going on holiday before marriage or dating exclusively as a couple before marriage (the latter another recent invention of 20th century western society). However, certainly if one reads all the research, overwhelming the data shows that couples that lived together before marriage have a higher chance of divorce, creates and re-enforces different views of the relationship (women often view as it as step towards marriage, whereas men often view as a step before making a commitment). So largely secular research aligns with the Bible on this area.
2. Cohabitation before marriage isn't a sin, but sex before marriage definitely is. And given the fact the latter will inevitably follow from the former, cohabitation should be considered a sin by straightforward implication:
This would be my own viewpoint (personally, I cannot and never have been able to, conceive of one without the other, even if the couple are known to have separate bedrooms). I would argue for the sex before marriage aspect alone, a mature Christian shouldn't live together before marriage. And just like any other sin (being divisive, misusing money, practising witchcraft, whatever) that was being done by someone who claimed to be a committed Christian, this would be case for loving rebuke, and if this was someone with leadership role (elder, small group leader, youth pastor) there would be a case for church discipline. But that's my perspective, very open to hear from those who may be able to address this from a more pastoral angle?
3. Cohabitation before marriage isn't a sin, but it still devalues marriage as a public leaving and cleaving, and therefore in general shouldn't be encouraged, (and also could be unloving to less mature believers who may be unsure themselves)
I've seen first hand the impact this had on me as someone growing up a youth group, from the youth leader who lived together and then had a child outside of marriage. This was not helpful to a number of us who had mixed experiences of that group, and this only added to the confusion.
4. There may be some situations where cohabitation before marriage is the only option (e.g: both partner is soon to be made homeless, and/or come from an abusive home situation, so this is the lesser of too evils).
5. If the couple are newer Christians, and it is revealed they are sleeping together, cohabitation may be a lesser of two evils, since it resembles marriage closer than sleeping together. whilst continuing to maintaining two separate households
6. Its unloving to show approval of couple own self-understanding of what marriage (that cohabitation/having shared assets is a full equivalent to marriage) where it differs from a Biblical understanding (to use an analogy a transgender person who thinks they are a different gender to their biological sex should be loved, but still not have their own self understanding approved)
Concluding comment:
Just to say apologies if this may come across as quite black and white. I'm High Functioning Autistic, and if some Christians own temptation might with over-relativising particular aspects of Christianity that are black and white, to affirm all people's perspectives/feelings, I sometimes find myself leaning towards the opposite end (i.e: my own temptation is making everything black and white for a sense of personal safety/control, reducing uncertainty).