More than a billion people in the world go to sleep each night without reliable shelter. But a pair of companies working on solving that believes their model of quickly 3D-printing a one-story house could not only provide merely a roof over the head, but a genuinely great place to live. I took a walk around a demonstration house ... built by Icon, a construction firm, and New Story, a non-profit that sets up housing in the developing world. Later this year, the project will head to El Salvador to build some test homes, with the view to begin work on a community of 100 houses in 2019. If it does work it could literally change how shelter is created," said Brett Hagler, chief executive and co-founder of New Story. Like small-scale 3D-printing, the system works by slowly adding material, layer-by-layer. In this case, that material is mortar, similar to concrete. The El Salvador project will ... aim to build 100 homes, financed by mostly Silicon Valley-based donors. The houses will not be a hand-out, however. "The families agreed to a no-interest, no-profit mortgage that they will pay over about 10 years," explained Mr Hagler. That works out at about $30 a month. According to the country's economics ministry, the average monthly wage in rural El Salvador is around $360. "That money does not come back to us," he added. "It's kept in a community fund." The fund will go on to pay for more homes in future, or maintenance on existing structures. Mr Hagler said the mortgage model will foster "respect and dignity" within the community.