The ability to create very affordable, but quality shelter/housing. Cheap & effective materials, a solid, reusable design, small footprint, easily replaced components, etc.

Off the top of my head, this might look like:

Cement block housing, very limited plumbing (with ability to have a pitcher pump at the sink), composting toilet, some kind of simple fold-out cot/bed/hammock, small furnace/woodstove?, etc.

Make housing so simple & affordable that every single person can have a safe place to stay, even if that place is without luxury.