I disagree. Shelter is only bound by what it costs to build a new shelter. This is where regulation has had a dramatic impact over time. Nobody would ever allow you to build a pioneer log cabin as a primary dwelling these days, even though the costs would be minimal to erect such a structure. There are very strict standards that must be adhered to in order to build shelter now.

Land is also necessary on which to build said shelter, but vacant lots in rural areas can be had quite cheaply. Lots of places where an acre, which is more than plenty to build shelter on, won't fetch much of anything dollar wise. That is, where regulation allows you to actually buy a small lot in a rural area from the surrounding landowner. Regulation often makes doing that very difficult.

The problem, at least not in a huge country like the USA, isn't population. There is incredible amounts of unused space ripe for housing someone. The problem is being able to build shelters for those people without getting caught up in red tape.