Rationalists like to live in group houses.  We are also as a subculture moving more and more into a child-having phase of our lives.  These things don't cooperate super well - I live in a four bedroom house because we like having roommates and guests, but if we have three kids and don't make them share we will in a few years have no spare rooms at all.  This is frustrating in part because amenable roommates are incredibly useful as alloparents if you value things like "going to the bathroom unaccompanied" and "eating food without being screamed at", neither of which are reasonable "get a friend to drive for ten minutes to spell me" situations.  Meanwhile there are also people we like living around who don't want to cohabit with a small child, which is completely reasonable, small children are not for everyone.

For this and other complaints ("househunting sucks", "I can't drive and need private space but want friends accessible", whatever) the ideal solution seems to be somewhere along the spectrum between "a street with a lot of rationalists living on it" (no rationalist-friendly entity controls all those houses and it's easy for minor fluctuations to wreck the intentional community thing) and "a dorm" (sorta hard to get access to those once you're out of college, usually not enough kitchens or space for adult life).  There's a name for a thing halfway between those, at least in German - "baugruppe" - buuuuut this would require community or sympathetic-individual control of a space and the money to convert it if it's not already baugruppe-shaped.

Maybe if I complain about this in public a millionaire will step forward or we'll be able to come up with a coherent enough vision to crowdfund it or something.  I think there is easily enough demand for a couple of ten-to-twenty-adult baugruppen (one in the east bay and one in the south bay) or even more/larger, if the structures materialized.  Here are some bulleted lists.

Desiderata:

• Units that it is really easy for people to communicate across and flow between during the day - to my mind this would be ideally to the point where a family who had more kids than fit in their unit could move the older ones into a kid unit with some friends for permanent sleepover, but still easily supervise them.  The units can be smaller and more modular the more this desideratum is accomplished.
• A pricing structure such that the gamut of rationalist financial situations (including but not limited to rent-payment-constraining things like "impoverished app academy student", "frugal Google engineer effective altruist", "NEET with a Patreon", "CfAR staffperson", "not-even-ramen-profitable entrepreneur", etc.) could live there.  One thing I really like about my house is that Spouse can pay for it himself and would by default anyway, and we can evaluate roommates solely on their charming company (or contribution to childcare) even if their financial situation is "no".  However, this does require some serious participation from people whose financial situation is "yes" and a way to balance the two so arbitrary numbers of charity cases don't bankrupt the project.
• Variance in amenities suited to a mix of Soylent-eating restaurant-going takeout-ordering folks who only need a fridge and a microwave and maybe a dishwasher, and neighbors who are not that, ideally such that it's easy for the latter to feed neighbors as convenient.
• Some arrangement to get repairs done, ideally some compromise between "you can't do anything to your living space, even paint your bedroom, because you don't own the place and the landlord doesn't trust you" and "you have to personally know how to fix a toilet".
• I bet if this were pulled off at all it would be pretty easy to have car-sharing bundled in, like in Benton House That Was which had several people's personal cars more or less borrowable at will.  (Benton House That Was may be considered a sort of proof of concept of "20 rationalists living together" but I am imagining fewer bunk beds in the baugruppe.)  Other things that could be shared include longish-term storage and irregularly used appliances.
• Dispute resolution plans and resident- and guest-vetting plans which thread the needle between "have to ask a dozen people before you let your brother crash on the couch, let alone a guest unit" and "cannot expel missing stairs".  I think there are some rationalist community Facebook groups that have medium-trust networks of the right caution level and experiment with ways to maintain them.

Obstacles:

• Bikeshedding.  Not that it isn't reasonable to bikeshed a little about a would-be permanent community edifice that you can't benefit from or won't benefit from much unless it has X trait - I sympathize with this entirely - but too much from too many corners means no baugruppen go up at all even if everything goes well, and that's already dicey enough, so please think hard on how necessary it is for the place to be blue or whatever.
• Location.  The only really viable place to do this for rationalist population critical mass is the Bay Area, which has, uh, problems, with new construction.  Existing structures are likely to be unsuited to the project both architecturally and zoningwise, although I would not be wholly pessimistic about one of those little two-story hotels with rooms that open to the outdoors or something like that.
• Principal-agent problems.  I do not know how to build a dormpartment building and probably neither do you.
• Community norm development with buy-in and a good match for typical conscientiousness levels even though we are rules-lawyery contrarians.

Please share this wherever rationalists may be looking; it's definitely the sort of thing better done with more eyes on it.