All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
It is not your duty to pay Zakah on those plots of land as long as you did not buy them with the intention of trade. The fact that you may sell these lands when the need arises does not render them articles of trade. Ibn ‘Uthaymeen was asked about a man who possessed a piece of land and was uncertain about what he wanted to do with it; he did not reach a final decision whether to sell it, build a building on it, rent it, or build a house on it and live on it: Is he obliged to pay Zakah after having possession of it for one lunar year? He answered, “No Zakah is due on this piece of land as long as the owner is not resolved on selling it for profit. Zakah is not required in this case because he is hesitant about it. No matter how insignificant such a hesitation is, the piece of land is not considered an article of trade and thus does not require Zakah.” [Majmoo‘ Fataawa Ibn ‘Uthaymeen]
He was also asked, “If the person is uncertain and wonders whether he should sell it (the piece of land) for profit, keep it, build a building on it, does such a land require Zakah? The answer reads "No, it does not require Zakah because in origin, Zakah is not required unless the person intends to sell his property for the purpose of trade.”
As for the revenues from the building you have leased, Zakah is required on whatever remains of it after having had possession of it for a lunar year as long as it reaches the due Nisaab (minimum amount liable for Zakah). Zakah is not required on whatever part of the revenues from rent you spent during the lunar year. For example, if you spend 10,000 and have 13,000 left, Zakah is only required on the 13,000, and you should pay 2.5 % of the 13,000 only.
Allaah Knows best.