Two Amsterdam tenants had great success in reporting their landlord to the Huurcommissie rent tribunal for charging them exorbitant rent. The Huurcommissie ordered the landlord to lower the rent for a small, leaky, drafty room on Keizersgracht from 1,950 euros to temporarily 95 euros per month. The rent for another room in the same building was lowered from 1,200 euros to temporarily 93 euros.
You just showed why prices would go up if rent were taxed that way (and why the prices are so high at the moment). No one thinks a small flat is worth 1500€/month (or what have you), but they need a place to live near their job. So they’ll pay whatever it costs. Same with deregulated health care, like in the US.
Also, for what it’s worth. Even without the construction costs and without any profit margin landlords must pay for home repairs. I don’t think 5% would even cover that