To me sustainable means simple, economical, robust, little bit like wabi-sabi. Sustainable is something that we can keep doing for a long time with success. Without harming the environment. And it does mean more simple, smaller houses. Yes, better insulated, but not obsessed with insulation. Air-tight to exactly the level required, not more, and certainly not a Tesco bag in which we cannot even breathe without a machine, just like darth wader. 
It means a lot of thought has to go into the layout. This is generally what architects do. We will provide the technical details so that it is built as per design. It is a lot more work for everyone, but the end result is something which will stand and not go out of fashion for a 100 years. Or anytime soon. Just like wabi sabi. Not perfect, but sustainable.

It means designing down to the last detail to make sure everything fits together perfectly.

Sustainably used space means exactly that: nothing is wasted. If building regulations allow, bunkbeds or storage can be formed in attic spaces.

A layout which does not have much to spare at all . . yet, it has a lot to offer. The house is designed to be naturally and organically expandable as the family grows. It means upwards (large attic space ready for conversion to 2 bedrooms) and to the rear. If orientation dictates, then to the side as well. It is like a simple Lego. Build what you can afford now, add onto it later on, as you need.