Sustainable garden and landscape design are aspects of sustainable urban design. Since sustainablilty is a relative concept and not an absolute concept, it makes no sense to call one design 'sustainable' and another 'unsustainable'. We can only compare two designs and pronounce one 'more sustainable' and the other 'less sustainable' according to defined criteria. Every roof design, for example, will transmit energy, discharge water and have a limited lifetime. In varying degrees, greener roofs will: transmit less energy, discharge less water, last longer, absorb more noise, absorb more dust, absorb more CO2, generate more electricity, re-cycle more organic material. Compromises will have to be made and improvements will always be possible, so that no roof can be defined as SUSTAINABLE in an absolute sense.
Relative sustainability, defined as the difference between a more sustainable city and a less sustainable city, depends on the principle of having fewer inputs and few outputs - or energy, materials, waste products etc. See top diagram, left.
A sustainable city has fewer inputs and fewer outputs
Sustainable cities will have vegetated roofs and walls which generate electricity from solar power.
Old cities have unvegetated roofs, which made need conservation
Sustainable cities should not be dreary collections of walk-up appartment blocks