Yes, okay, but ... what does this have to do with sustainable building? We can explain this step by step, starting with the first: migration.
Every day, 180,000 people migrate from rural areas to go live in cities.
By 2030, it is estimated that 6 out of 10 people will live in cities.

In fact, not exactly "cities" but in the poverty belts surrounding them: the shanty towns. This distorted world map shows where the majority of these populations are concentrated and therefore where more people are forced to leave rural areas: Black Africa, India, Pakistan, Bangladesh, China ...

They are currently one billion living in slums, and this number is increasing exponentially: in 2030 there will be two billion.

In cities like Mexico, Lagos or Calcutta, the slums have formed impressive extensions of the city (the size of small countries) within which full employment is no longer possible; unemployment rates are generally over 20% of the active population.

Working:

The percentages of unemployment in Spain: 22% (1996), 11% (2004), 9% (2005), 8% (2007-8), 20% (2009), 27% (2012) .. . Screening for 2015 is 25%. If we begin to understand that this is reality and not a passing crisis, it is reasonable to think we will not see rates below 20%.

The graph shows the importance to visualize the situation as a whole: even in the euphoric years just before the crisis in 2007-8, Spain never got down to 8%. This percentage signaled the end of the industrial era, after which the workforce is inexorably replaced by machines and automated processes, and unemployment will continually rise.

This gives us a new perspective: an "unstoppable" 8% unemployment and climbing, plus other circumstances dependent on the economic cycle For example, China is growing apace, but few people know that is bound to grow annually at least 5% if they want to maintain full employment.

In short, we are practising a model that does not serve us; we must keep running, only to remain in the same place. (Source: Surveys MTAS)

These city-slums pose a serious health problem. The lack of sewer cover over large roofed areas results in infiltration of sewerage into the soil when rainwater collects in the 'streets', polluting aquifers and attracting waterbourne diseases to the slums.

The problem of slums is unstoppable until their origin is analysed: the rural-urban exodus. Most newcomers are no longer migrants seeking a better future, but economic or environmental refugees (about 25 million), which can no longer live as they have for millennia.

(Img: permaculture.org.au/2009/11/09/the-rising-tide-of-environmental-refugees/).

These people are so poor they cannot access conventional homes and cannot pay bills for water, electricity or rent. This system of the costs of services is therefore not applicable to slum cities. A scenario with high rates of permanent unemployment is a fertile ground for crime, illegal work and social disintegration.
Stock Dharavi, in Mombay, the largest city-slum in Asia, where between 600,000 to 1 million environmental refugees live within 1.7 km2. A toilet is shared by hundreds of people and the density is 2-3 persons/m2.
In Mombay, 55% of the population live in shanty towns, which occupy 6% of the land.

(Img: www.tampabay.com/news/article983546.ece)

Therefore the option of having decent housing in such a situation is conditional upon:

- Discouraging use conventional high initial cost materials (cement, steel, glass ...)
- Being reversible rather than permanent.
- Finding solutions to exclude sewage from the streets, having clean aquifers and installing street lighting (remember that there are no customers or budget).
- Minimising the entry of supplies (butane, charcoal, coal ...) and inefficient use of nonrenewable resources (included the inevitable illegal connections to the electricity grid) which generate more pollution and noise.
- Preserving the active roles of the newcomers, encouraging their accustomisation to survival and daily work and preventing the entry of passive roles with low morale and discouragement.
- Lowering high blood pressure due to insecurity, overcrowding and frustration. Enhancing sanitation, shelter, privacy and providing the opportunity for newcomers to apply the resources they have acquired in the field (initiative, hard work, etc.) to building houses and worthy community service, something to feel involved with and proud of.

The picture shows the% of the urban population living in shanty towns.

(Img: //en.wikipedia.org/wiki/Slum)

Without access to commercial construction materials and licences and being unable to afford builders, the only solution left is ...

The solution is not conventional, but the alternative is clear: if there are no customers, firms will not invest in services, municipalities can not accommodate an infinite number of refugees, there is no other way out.

(Img: www.orchardpropertyservices.co.uk/index.php?page=self-build)

This neverending exodous reminds us that, as long as we do not remember where the origin of rural-urban migration is, we are just gaining some time, but not solving the problem: in a scenario of high structural unemployment, cities can not absorb labor force indefinitely.
People only migrate when conditions for life no longer exist ...

(Img://emilioutges.blogspot.com/2010/03/inmigracion-emigracion-y-madre-patria_24.html)