Like New Orleans, Rotterdam lies below sea level (from 6 to 18 feet) at the mouth of an enormous lowland delta. Paula Verhoeven, director of sustainability for Rotterdam, explains that her city is focused on water management in the central area. The municipal government is committed to densifying its urban core rather than sprawling beyond the current limits. A study by the Dutch General Research Institute showed that to make Rotterdam livable with more density, it would need to “green” the city.

Rotterdam has made a major effort to reduce the amount of hard surfaces by promoting features such as green roofs, according to Verhoeven. “Green roofs have several benefits: they slow and filter stormwater, help keep the building cooler, and reduce the urban heat-island effect,” she says. The urban heat-island effect results from heat-absorbing building materials and the canyonlike forms around tall buildings. The trapped heat can raise the ambient temperature, making buildings more difficult to cool and intensifying the ill effects of a warmer climate.

The city is working with the firm DeltaSync on projects that explore the potential of large-scale floating extensions to the urban area. (If this sounds like science fiction, consider the Uros people, who centuries ago built a constellation of floating villages on Lake Titicaca, Peru.) The scheme would include agricultural areas (yes, the Dutch have developed a saltwater-tolerant potato) and urban developments. The design team completed a 3,000-square-foot domed prototype that serves as an exhibition structure in Rotterdam's harbor.

Floating houses have been around for centuries, but a bevy of Dutch firms have come to specialize in water-borne architecture. Amsterdam's +31 Architects is working on several residential projects that promote urban densification without taking up valuable real estate. The Watervilla de Omval, in Amsterdam's harbor, uses a hollow reinforced-concrete shell to give the house its buoyancy. According to +31 principal Jasper Suasso de Lima de Prado, the house connects to electrical, water, and sewer lines through pliant tubes that flex as the home rises and falls with the tides.