If he is hesitant about whether he wants to sell the land, then there is no zakaah on it
Question

I have a piece of land; when I bought it, my intention was 70% that I would sell it. Is zakaah due on it when I have the intention 30% to build on it and not sell it?
Praise be to Allah.So long as you do not have a firm intention to sell it, then no zakaah is due on it, because the basic principle is that it was bought to be kept, so it does not become like trade goods except when you form the firm intention to sell it. But if you are still hesitant, then no zakaah is due on it.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: There is a man who has a piece of land, and his intention regarding it was not certain. He does not know whether he wants to sell it, develop it, rent it out, or live on it. Should he pay zakaah when one full Hijri year has passed?
He replied: No zakaah is due on this land at all, so long as he has not firmly decided that he wants to sell it. So no zakaah is due on it, because he is unsure, and if one is hesitant even one percent, no zakaah is due on it.
End quote from Majmoo‘ Fataawa Ibn ‘Uthaymeen (18/232).
And he said: If a person is hesitant and says: By Allah, I do not know whether I should sell it or keep it – for example, he has a piece of land and he says: I do not know whether I should sell it or keep it or build an apartment block on it, is zakaah due on it or not?
The answer is: no zakaah is due on it, because the basic principle is that zakaah does not become obligatory until one has a firm intention that he wants to sell it.
End quote from al-Liqa’ ash-Shahri (3/5).
And Allah knows best.