In the discussion about the number of units it was said that the 20-30 figure came from Mark. I didn’t want to interrupt for such a detail, but the number actually comes from the manual “Creating Cohousing” (saved in the “Books and manuals” folder | internal link).
The relevant text is on p. 248 under the heading “Community size”. Here’s an extract:
The best size and number of households seems to be one of the biggest challenges facing cohousing in America. Cohousing communities in Europe have demonstrated that there is an optimum size for a cohousing community. If a community is too big it will have an institutional feel and will lack the intimacy required to make its residents feel as if they have a stake in its success. If a community is too small, it will resemble a large family rather than a neighborhood of actively engaged households. We have studied and designed communities of many different sizes, and have divided them into three different sizes by social and physical characteristics:
Small Cohousing Developments (8 to 15 households). Small cohousing developments are simpler to organize and require less land, making it easier to find good sites. Small projects are also less likely to attract neighborhood opposition, and the development budget is generally within the financing capacity of small developers or even the residents themselves. Managing a small cohousing development is less complicated and less formal, because fewer people are involved; most adults participate in what is as much a discussion as a meeting.
On the downside, many development costs are fixed, no matter how large the project. Project management, for example, costs the same for 15 or 30 houses — so it is often easier to control costs on larger projects. A small community requires more compatibility, allows less diversity, and requires a greater community commitment from each individual. If there is a serious disagreement among residents, it is more common for a household to move out (whereas in larger communities
they can just avoid each other for a while). In addition, the common facilities usually require a larger financial investment per household.
When we asked about the issue of size, residents of cohousing developments with 15 or fewer households often commented, “It could be a little larger.” “At only 12 or 13 households, you have to work at it,” was a common sentiment.Medium Cohousing Developments (16 to 25 households). When asked about size, residents of medium-sized cohousing developments usually thought their own was close to perfect. “Twenty houses is just right,” announced Alice from behind the stove in the common house, “because you only have to cook once a month.” There are 32 adults in her community, and with two people cooking dinner four nights per week, each adult cooks just once a month. “It’s small enough to know everyone well,” is a common statement from residents.
To make a case for the medium size is to make a case for cohousing itself — large enough to have extensive shared facilities, but small enough to be easily managed by direct democracy. This size can more easily accommodate variations in individual schedules; it’s not a big deal if several people miss a meeting or a workday, but when people are there, they have direct input. While decisions are still made through the consensus-seeking process, their implementation can be distributed among the entire community. Although decision-making becomes more formalized than in small communities, it easily includes everyone.Large Cohousing Developments (26 to 35 households). A large community allows for greater diversity of ages and family types, and common facilities can be more extensive and affordable through economies of scale. The participation of a nonprofit organization is more likely to factor in with large communities, thus allowing for government subsidies. But planning approvals and financing arrangements are also more complex for large projects, which are also more likely to attract neighborhood opposition, further slowing the approval process. In fact, we do not recommend that a resident organizing group attempt to build a community of this size without collaborating with an experienced developer and a very experienced architect.