Plan Bay Area has already drawn several legal challenges, and the debate could spread nationwide if, as may happen, it becomes a model for regulators in other parts of the country.

Owning a single‐​family home has long been part of the American dream, but Plan Bay Area embraces a dramatically different vision of the ideal community: crowded rows of high‐​rises and mass‐​transit platforms.

Population density in the region’s urban areas would increase by 30 percent during the next two decades under the plan. Nearly 80 percent of all new housing and 62 percent of new jobs would be located in just 5 percent of the region’s surface area.

Planners admit this will make single‐​family housing in the already high‐​priced Bay area even less affordable.

To be sure, the plan isn’t the first attempt to herd families into condominiums and apartments. Since at least the 1970s, urban planners around the country have argued that the single‐​family‐​home lifestyle results in people driving too much, which supposedly wastes energy and pollutes the air. Thus, 17 years ago, Portland, Ore., adopted a scheme to reduce the share of residents living in single‐​family homes from 65 percent to 41 percent. In some neighborhoods, if a house burns down, it can be replaced only with an apartment structure.

Even if it’s not without precedent, Plan Bay Area could still be revolutionary because of the rationale behind it. It could help spur a nationwide movement for high‐​density “transit‐​oriented” development — in the name of reducing global warming. The federal government has signed on. The Obama administration has told metropolitan areas to include land‐​use regulations in the transportation plans that federal law requires them to update every five years. Washington is also giving communities “livability grants” aimed at promoting high‐​density development.

As a result, cities that are far removed from San Francisco in a political sense — Des Moines, Iowa, and Lafayette, La., for example — are considering similar land‐​use restrictions.

Advocates argue that the demand for single‐​family homes is about to drop as retiring baby boomers and up‐​and‐​coming millennials will prefer to live in mixed‐​use neighborhoods with high densities and easy pedestrian access to stores and entertainment.

This claim isn’t supported by people’s actual behavior. The vast majority of population growth continues to be in low‐​density suburbs. Surveys of millennials show that more than three out of four aspire to live in a single‐​family home with a yard.

The data also show that crowding people together isn’t really effective at reducing greenhouse‐​gas emissions or addressing other urban concerns. Population densities in the San Francisco‐​Oakland‐​San Jose urban areas have already grown by nearly 60 percent since 1990, yet per‐​capita driving still has increased.

Even under planners’ most optimistic projections, Plan Bay Area will have negligible effects on carbon‐​dioxide emissions: The draconian land‐​use changes will reduce emissions by only about three‐​quarters of 1 percent.

In contrast, improved fuel economy, already mandated by the state of California, is projected to reduce per‐​capita emissions by more than 30 percent in the coming years.

Other transportation programs, such as van pooling and giving drivers incentives to use electric cars, are expected to reduce per‐​capita emissions by nearly 3 percent more.

As Americans consider the future growth and development of their communities, the Plan Bay Area debate should offer a message of caution. Forcing people to live in crowded “stack and pack” housing developments curtails freedoms without substantially curbing greenhouse gases.

There are many ways to reduce emissions that are genuinely cost‐​effective, some of which — such as making cars and homes more energy‐​efficient — could actually pay for themselves in the long run. In contrast, decreeing radical lifestyle changes for average Americans is expensive, intrusive and ineffective — squandering the political and financial capital needed for real improvement in environmental quality and our quality of life.