Bacteria release enzymes that break down tissue into component parts like amino acids, and eventually, the nitrogen-rich molecules bind with the carbon-rich ones, creating a soil-like substance.

Temperatures reach around 140 degrees, often higher, and the heat kills common pathogens. Done correctly, there should be no smell. Bones also compost, though they take longer than tissue.

Ms. Spade has designed a building for human composting that aims to marry the efficiency of this biological process with the ritual and symbolism that mourners crave. Each Urban Death facility would be centered around a three-story vault that she calls “the core.” Loved ones would carry their deceased, wrapped in a shroud, up a circular ramp to the top.

There, during a “laying in” ceremony, mourners would place the body inside the core, which could hold perhaps 30 corpses at a time. Over the next several weeks, each body would move down the core until the first stage of composting was complete. In a second stage, material would be screened, along with any remaining bones, and the compost would be cured.

Ms. Spade estimates that each body, combined with the necessary materials such as wood chips and sawdust, would yield enough compost to fill a three-foot cube.

Weeks or months later, survivors could collect some of the compost to use as they saw fit, perhaps in their garden or to plant a tree. Ms. Spade foresees the rest going to nearby parks or conservation lands. Each human composting would cost about $2,500, a fraction of the price of conventional burial, Ms. Spade estimates.

She hopes to build the first facility in Seattle, then to develop a template that other communities can use for locally designed facilities. “Like libraries,” she said.