It's a great question! On one hand, minimal standards for space, damp-prevention, ventilation and toilets should be applied and developed, but also alternative methods of construction should be considered and encouraged, especially factory produced low cost housing.
By implementing very effective Selective Credit Control Measures, ensuring flow of created credit proportionately to all necessary and sustainable sectors of the economy and diverting it away from the wasteful and speculative uses.
In our work we also don't simply look at providing "sustainable housing" but also look at the bigger picture of creating "sustainable economies" linked to housing: there is no point creating homes if there are no opportunities for entrepreneurship, incubator support or other forms of sustainable job creation to complement other forms of work.
This may be wandering off topic slightly, but "sustainable" is such a large umbrella term for society.