To take deprciaiton on any item it has to be placed in service, or available to be placed in service (If you can't find a renter).
So if are half finished with a project and not renting the space, you have to keep the costs and then list it as an asset when finished which sounds like 2019.
Now if you did part of it and it was rented out after that, then you would list it as an asset in 2018, and then another asset in 2019 when it was completed. So it would be 2 different assets totaling 100% of the project.