How to calculate adjusted cost basis for an investment property?

Hi, 

  1. I bought a house in 2016.

  2. Between 2016 and 2019, I rented it and declared the rental revenue and expenses on a Form 1040 - Schedule E

  3. In 2019, I demolished the house and started the construction of 2 units on the lot

  4. From 2019 to 2024, construction is ongoing. I incurred $1.3M of expenses with 99% of receipt for it. I did not declare anything related to the property during those years as there were no income.

 5) In 2024, I sold one unit.

My question are the following:

a) to calculate the gain/loss now that one unit is sold what basis should we take?

Fake numbers for illustration:

Price of purchase in 2016 = $300K

Adjusted basis at the end of the rental period = $350k (improvement, depreciation...)

Once the house is demolished, do we start from this basis of $350k to calculate the gain/loss?

Or do we need to know the price of the land only as the house is demolished? Then, should we use the FMV of the land as of 2019?

b) can I wait to sell the other unit in 2025, or do I need to declare the sale in 2024 even if I keep incurring costs on the other unit?

c) I think it would fall under capital gain, but just wanted your opinion. 

Thank you so much in advance for your help,