Building Expansion

You can't.

 

(6)Qualified improvement property

(A)In general

The term “qualified improvement property” means any improvement made by the taxpayer to an interior portion of a building which is nonresidential real property if such improvement is placed in service after the date such building was first placed in service.

 

(B)Certain improvements not included

Such term shall not include any improvement for which the expenditure is attributable to—

(i)

the enlargement of the building,

(ii)

any elevator or escalator, or

(iii)

the internal structural framework of the building.

 

Section 179

 

 

So I need to add the cost of the expansion to the historical cost of the building?

You need to create an improvement to the building as a new asset and start depreciating it over its useful life.