ALL costs to complete the project are totaled and will comprise the cost basis of the asset which starts being depreciated when placed in service ... just because it took more than one tax year doesn't change that fact that nothing is taken until it has a business use.
Read up on this in the IRS Pub 527, pub 551 & pub 946.
https://www.irs.gov/pub/irs-pdf/p527.pdf
https://www.irs.gov/pub/irs-pdf/p551.pdf
https://www.irs.gov/pub/irs-pdf/p946.pdf
