Set in law a limit on how many properties an individual landlord / leasing company can own and lease. The law would need to take into account individuals simply creating another company to own more property and be able to stop this practice.
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As rent control is not being implemented in any meaningful way, landlords are able to continue to increase rents as the demand for housing grows and available property numbers remain stagnant. The more property owned by less people means less competition and increased rents. People need a home, its a simple as that, yet they are paying far more in rent, on average, than they would if they could purchase a property, of which there are few and even less are affordable.