I believe that by the middle of the century, it could be possible to have a home for every person in the world, as well as every subsequent person who will be born between now and the middle of the century. Here is the plan, given in approximate numbers, as in a Fermi calculation. Fermi calculations are named after Enrico Fermi, a physicist from the Manhattan Project who was able to estimate the force of the atomic bomb tests by watching how far a sheet of paper was blown by the blast wave. He did this by using extremely rough numbers, which didn't get any degree of precision, but they were typically accurate at least to the general size of the answer.
Let us have a total workforce of, say, 20,000,000 workers (about 1/400 of the world’s current population -- if there needs to be more workers to actually pull this off, there are still lots and lots of people who would be glad to help build the structures where they will ultimately end up living), simultaneously building a bunch of 10-story buildings, with, say, 20 apartments per story, and, say, 3 people per apartment. This means that there are about 600 people living in each apartment building. There are to be 16,000,000 buildings total, each taking up about 140 ft by 140 ft square of land, give or take. Workers are divided into 200,000 teams. Each team has 100 people. All teams are working simultaneously. That is the part that actually makes this plan work. Let us say that 1 team can make 1 building as described above, in about, call it 3 months, I assume. Therefore, in 20 years, 1 team can make about 80 such buildings. 80 times 200,000 teams equals all 16,000,000 buildings constructed in 20 years. 16,000,000 times 600 residents per building equals about 10,000,000,000, the estimated world population by the year 2050. If project starts in 2020, it will end in 2040. 10 years later, the buildings will be full.
But how much land is required? Each building is around 140 by 140 feet, which is around 20,000 square feet for one building. Multiplied by 16,000,000 buildings, that equals around 300,000,000 square feet. One square mile is around 25,000,000 square feet. Dividing 300 million by 25 million gives a mere 12 square miles. And remember, this can still be spread over the entire world, so each individual section of this largest international apartment chain ever built would only take up a rather small area.
The fact is that even the entirety of the human race, taken together, and allowed space in which to adequately live, is still minuscule compared to the entire Earth, which is why this calculation works out. Now, there is just one matter left. How much will this cost? This is probably the biggest difficulty, and I have no idea how to even estimate this value, because I do not know what apartments are typically made of, or how much those materials cost. And even if I did, who knows how the prices might change over the course of the construction?
Of course, one could just make a very rough estimate, as that is the spirit of a Fermi calculation. Assuming a certain price for each square foot of living space, multiply it by 20,000 to estimate the price of one floor, multiply it again by 10 to get the cost of all 10 stories, and finally multiply it by 16 million, which, if each square foot costs only 1 single dollar, comes out to 3,000,000,000,000 dollars, which is 3 million millions. So it is in the realm of trillions of dollars. It sounds like a whole lot of expense, but bearing in mind that this is a global-scale construction project, it might make the sheer amount of expense not seem so shocking. And remember, this is solving a problem that will affect everyone alive in the world, all 10,000,000,000 projected people by the time it is complete, which amounts to 3,000 dollars for each person housed, or roughly in the thousands of dollars for an individual. And remember, this does not account for any preexisting houses or homes, and it is all very rough guesswork anyway.
So that is it, my plan.