Let’s fix a housing crisis by having houses be used.

I doubt this is the magical unique problem everyone claims it is, but “oh no think of people’s investment” when people are sleeping outside is valuing property over people.

Housing vacancy is essentially a red herring when it comes to the housing crisis. Vacancy rates are still low overall, and reducing vacant housing would barely make a dent in the issue. The real issue is that there isn't enough supply for a given level of demand (i.e. how many people there are who do or would like to live there). Cities have added way more people than they've added housing units over the past several decades.

So would you say the same applies for property which is being underutilized? Throw out the rich people living in 5 bedroom houses? Throw out the grandmas living in the house they once had a family and children in? Mandate minimum number of persons per square foot?

Hmm, I’m fairly certain we have some laws giving squatters rights if they live in some housing long enough without opposition.

It's not uncommon for cities to pas laws that aim at the reduction of unoccupied spaces amid housing crisis. Actually, most European capitals do this either by raising taxes on empty housing or by partially legalizing squatting.

Londongrad is an exception to what is otherwise a rule.

Houses that are vacant for a long period attract higher council tax in London and the whole of England.

Of course you may argue that the difference is small in relation to multi-million pounds properties, but that is a more general issue with the council tax's range.

America fixed it's British Colony issue by doing just that.

I'm not sure society is going to break down if a few empty rich investor properties actually get used.

squatting is a "solution" to empty houses. Its not a good solution but it certainly draws attention to the issue.