Is Zakat obligatory on the proceeds from the sale of a plot of land that was purchased for the purpose of building, but then left for twenty years for the purpose of its value appreciating, upon its sale, or is the passing of a hawl (lunar year) on the sale proceeds a condition?

As long as you did not purchase the land with the intention of trading, you do not owe zakat on it for the past years. Zakat becomes obligatory only on its price after one Hawl (lunar year) has passed from the day you received it. This is the view of the majority of scholars (the الجمهور), which is the fatwa we follow. So, if you sell the land and receive its price, you should start a new lunar year count for that money. When the year is complete, you should pay a quarter of a tenth (2.5%) from it. Al-Mawardi al-Shafi'i mentioned in Al-Hawi that mere intention does not transform an asset into a trade commodity. This is the view of Malik and Abu Hanifa, differing from Ahmad ibn Hanbal and Ishaq ibn Rahawayh.