Long vs. Short Term Capital Gains after selling a commercial proj

[Rocco Addeo]

So I'm having a hard time determining if I need to pay long or short term capital gains on a new self storage facility. I started building it in 2015. I built 10 buildings in 2015 - 2016. Then in 2018 - 2019 I added 5 more buildings. As soon as I completed the last bilding I sold the facility. So my question is, do I pay Long term capital gains on the whole thing? Some Long and some Short...How do I calculate this mess? Thanks for any insight on this...BTW..I owned the land outright before i started.