One way to reduce our footprint is with flexible use of space.
This video shows a modular approach to the structure itself.
Moving to the inside, this MIT video shows a high-tech example that might inspire lower tech approaches as well.
Another take on a similar theme.
Here’s an article describing the idea of a flexible house as it applies to different life-stages, e.g., with and without kids at home.
This video focuses on transformable furniture (non-English):