How is zakat on money saved to buy a house for the past two years, after it reached the nisab, to be disbursed, and how is the nisab calculated for the coming year?

Zakat (definition) is obligatory on money saved for purchasing a house if it reaches the nisab (definition) (minimum threshold) and a hawl (one lunar year) passes on it. It is obligatory for anyone who has not paid Zakat for past years to pay it immediately. Zakat does not lapse with the passage of time. One must pay Zakat on the money for all delayed years, and if in doubt about the amount of money or number of years, one should act based on their best judgment. The questioner can designate the month of Rajab as the hawl for her Zakat if she is reasonably certain. As for calculating Zakat in the future, she has the choice between treating each saved balance as having an independent hawl, or paying Zakat on all the money she owns when the hawl passes on the first nisab she possessed. The latter is easier for her and more beneficial for the poor.