With my spouse we have a plot of 1000m2 where we built our house, then proceeded to divide the plot. We just want to sell the unbuilt part. The value of land to be used to calculate the real estate capital gain, zero?

No. If the sale concerns part of the land, the capital gain is determined by taking into account the fraction of the acquisition price relating to this part alone. The same applies to charges and indemnities increasing this price (BOI-RFPI-PVI-20-10-20 §130).