You can also try to find out the typical cost of building a house of size X, in your location, at the time you built it, and use it as a backup comparison to your spreadsheet.
The point is that you need to have reasonable, "typical" numbers, or numbers that are backed up with documentation. It is hard for the IRS to claim that an existing structure cost "$0" to build, in the face of no obvious effort to commit fraud, but if you paid more than typical it would be easy for them to state that the structure should have only cost $1k in 1988, not $100k, and deny it without backup.