Related: A Pattern Language
Epistemic status: Strong Anecdotal

I'm designing co-housing/co-living communities. The greatest benefit of living near your friends is that you can see each other all the time in low-stakes ways.

If you're living in the same house, this is very easy: you just hang out in the common area and say hi to everyone who goes through. If someone has time to chat then it's an easy way to get a little conversation, but it's not awkward if they are too busy.

This default doesn't really work well in a place where people don't live in the same house or share the same common space. I've spent time in communities where very close friends literally lived on the same block, but they didn't see each other more often than once a week or so, and all interactions require advance planning, because nobody is hanging out in a space where their friends cross through.

To avoid this failure mode, take the biggest or most welcoming house and make part of its common space a true community space. Unlock the door or gate, and let everyone nearby know that it's always ok to come hang out in that space, even if nobody is there. Ensure that you regularly pass through that space. Greet everyone who comes to it warmly.

Follow the intimacy gradient pattern: the unlocked space should be the most public area of that house, nearest the entrance. Try not to monopolize the entire common area of that house -- the people who live there need their own, more-private spaces when they aren't feeling as much like socializing.

You don't need to leave the door unlocked all the time. You should probably lock it while people are asleep. But I emphasize that any barrier against people coming over and hanging out whenever they feel like it will tend to make them dramatically less likely to do so -- subtle signals matter a lot here.

If you don't want to unlock it all the time, you can tell the community about the code to get in, or otherwise allow them to unlock the door on their own somehow.

This might take a while to start working. Some things you could try to go quicker:

• make a very warm announcement that everyone is super welcome at any time of day or night, no really we mean it, we love you all
• create a weekly hangout event, so people get used to coming over to the house regularly
• create an every-day super casual coworking or happy hour: a few people agree to spend at least 1 hour a day in the common space, even to work on their laptop alone if nobody else is there; quickly people will realize that there are always people around at this time, and it will be self-reinforcing.
• offer free beer or coffee :)