How is Zakat calculated on land purchased for investment and building upon, is it based on the original price, the current market price, or is it deferred until the actual start of the project?

If you purchased the land with the intention of building on it and selling the apartments, then it is treated as trade goods. It should be valued at market price at the end of every Hijri year, starting from the date of purchase. Zakat (definition) is obligatory on each partner, at a rate of 2.5% of their share, provided it reaches the nisab (definition). However, if you purchased the land to build on it for residential purposes or for rent, then there is no zakat on it. Overdue zakat remains a debt on one's conscience.