I suspect your term "build out" means it is either enlarging the building or involves the "internal structural framework of the building".  If that is the case, that disqualifies it as a "Qualified Improvement Property" (which qualifies for Section 179).
https://www.law.cornell.edu/uscode/text/26/168#e_6

That leaves 39 years as the only option, even if Congress fixes the "Qualified Improvement Property" to 15 years.