Astute observers say that the recent demographic notion of the nuclear family living in a detached home is mostly an historical anomaly of the modern Western world, an exception to the shared living arrangements commonly found in other times and places. It’s more typical of human culture, they say, to have several generations under one roof or at least on one premises, and to have closer physical proximity to one’s relatives and neighbors.

But U.S. households have bucked that trend for more than half a century. Relative affluence and greater mobility have loosened the ties that used to bind people to their extended families, villages and communities. Our neighborhoods, especially in suburban areas, have reflected that trend toward separation, but the advent of a new millennium has had a curious effect: a renewed interest in living more together rather than apart.

The factors behind this change are likely multiple and varied — an economic recession that has made sharing expenses smarter, if not necessary; baby boomers trying to care for aging parents who are living longer but can’t always stay independent; or just the attempt to balance global interconnectedness with a simpler tie to one’s local world.

Whatever the reasons for the cultural shift, it turns out that most of our current housing arrangements and residential zoning restrictions are decidedly unfriendly to it.

One lot, one home. It’s a simple way to build, and a simple rule to enforce. But it sidelines the potential for affordable housing in many urban areas, leaves homeowners and other residents with fewer options, and can even contribute to suburban sprawl. So what’s the alternative?

That’s the subject of Michael Litchfield’s book In- Laws, Outlaws, and Granny Flats (Taunton Press; $24.95; softcover, 218 pages; 800-888-8286; www.taunton.com). Litchfield, who has renovated a half-dozen homes, found himself living in a cottage that is the secondary home on a small California farm. It was a new arrangement for him, but not, he realized, for everyone.

The tradition of the in-law cottage, the basement apartment, the converted carriage house and other secondary living spaces is a well-worn path, Litchfield discovered, and he uses this book to show how others can travel it themselves.

He starts with the benefits, obvious or otherwise. For starters, the economics are pretty good. In some situations, the “accessory dwelling unit,” or ADU, can provide rental income and/or subsidize the mortgage payment for the owners and offer an affordable option for the tenant. In other cases, aging parents or other elderly relatives can live with or adjacent to family caretakers and still retain some independence, at a far lower cost than a structured senior community could offer. The space might also be used for a home-based business, reducing overhead costs that would come with an outside commercial lease.

In Litchfield’s view, the ADU option has little downside, but he does walk readers through the decision details just the same.

First, the definition: It means creating a living/sleeping area, bathroom, small kitchen and, usually, a separate exterior entrance. He talks about the financial and social considerations, how to evaluate your site/home for potential, and the newfound willingness of many city zoning boards to allow or even encourage ADUs in their communities. He even includes a quiz for readers to evaluate their tolerance for sharing their space with others.