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.
The answer to your question is summarized in the following points:
First, the pieces of land that you bought while they are still under development, if you bought them with a final sale contract without the element of Khiyaar (option of annulment, meaning that the sale contract cannot be canceled by either of the contracting parties), then the basic principle is that you are obliged to pay zakah on them because you own them. You should estimate the monetary value of the land after the passage of one lunar year on the capital that you bought them with, and then you pay their zakah, which is 2.5% of their value.
Second, the scholars held different views about deducting a debt from the wealth liable for zakah. The view adopted at Islamweb is that of the Maaliki scholars, which says that if you have other property that is kept for personal use (not for investment) and that you do not need, then you can make it a collateral for the debt and then pay zakah on the wealth that you have. If you do not have other property kept for personal use to serve as debt collateral, then you should deduct the loan from the wealth that is liable for zakah. For more benefit, please refer to fatwa 116828.
Third, if you intend to build a house on one of these two pieces of land and sell the other, then the scholars held different views regarding the liability for zakah in this case. Some held that you are not liable for zakah on either piece, and others held that you should only pay zakah on one of them and that you then choose which of them to pay zakah for. It is more prudent to pay zakah on one of the pieces of land in order to avoid the scholarly difference of opinion in this regard.
Allaah knows best.