Am I Islamically obligated to pay an additional amount for increasing the house area beyond what was agreed upon in the contract, knowing that the error in calculating the area was on their part?

If the price of the apartment was agreed upon based on its area, and the company made a mistake in its estimation, such that the actual area was larger than what was written in the contract, then the buyer has the option between paying the price difference or returning the apartment, unless the company waives its right. However, if the contract for the apartment was made based on its observed area without specifying a price per meter, then the company is not entitled to anything for the increase, because the area is considered a characteristic of the sold item. It is recommended to be lenient in selling.