Residential real estate represents 5% of the GDP (in the US), 25% of an individuals wealth, and 65% of the homes are owner occupied. A 20% tax even if only for property deemed underdeveloped would bankrupt a ton of people (myself included) and throw the entire market into chaos. Most people who were hit with this wouldn't be able to redevelop because of the expense, so they would be forced to sell. The market would get flooded and drive down the value. Investors would snatch everything up, redevelop, and start driving up rental prices. We need change but this would be horrible in so many ways.