Regarding the cost of building more social housing.
A lot of people need benefits because of lack of SH. Also, housing isn't an issue that exists in isolation. For example, housing issues massively affect health (and cost the NHS loads).
It is not the job of us, the taxpayers, to provide anybody in this country with a permanent home of their own. That is up to them.
Everyone is a taxpayer, and investing in decent SH is one of the best investments we could do.
No permanent homes for anyone not wealthy enough to afford one? Unstable lives create and increase social problems. Then people wonder why the NHS, social care, police, and the benefits system cost so much, and why there's a mental health crisis.
Also why our towns, cities, and villages are declining, and why there's more state dependancy (large transient populations means loss of community and support).
Whatever anyone thinks about 'feckless' parents, you can't want to see homeless children?
And what about disabled people? Nobody chooses to be disabled. People fleeing DV?
People in low paid jobs? Everyone can't all get higher paid work because the lower paid jobs still need doing.
The staying in larger houses thing. Lots of older and disabled people need extra space for carers or medical equipment.
The big thing though is there's not enough smaller homes, and the current downsizing policy unfairly penalises childfree people (who are not only taxpayers but often use less public services. For example, schools and maternity care). It's linked too to the issues discussed in this thread of 'feckless' parents, because the current system punishes the childfree.