Canada

Yup, I've always advocated for a punitive tax on owning more than one residential home. Start with corporations, 1% extra tax on the value of the property each year, increasing at 2x the rate of inflation each year. Two of three years after that, then apply it to privately owned second homes... Within 10 years, the annual tax on any second, third, fourth homes will be greater than any possible profits, and those houses will come on the market as various properties pass into not being profitable investments anymore.

And as others have said, use the collected tax for affordable housing. The best thing to do is for this money to be used to buy condos in existing buildings, and place families there, to prevent "ghettoization" of neighbourhoods.