Is Zakat obligatory on an amount designated for building a house that fluctuates unstably, and how is its Zakat calculated?

If the money reaches the nisab (definition) (minimum threshold) and a full hawl (lunar year) passes over it, zakat (definition) becomes obligatory. The muzakki (one paying zakat) has the choice between making each increase subject to its own separate hawl, or adding it to the original capital and paying zakat on the total at the end of the original capital's hawl. If the wife gifts her money to the husband, then he is responsible for its zakat. However, if she is a partner with him in building the house and has not gifted him the money, then she has a share, and she is responsible for its zakat if it reaches the nisab and a full hawl passes over it.