The IRS can challenge anything at any time for any reason. All income is deemed to be taxable unless you can prove otherwise.
Of course, the county has records of what you paid for the property in 2002, or a real estate agent through the multiple listing service.
I'm not clear on what you are selling, both properties or only the one you don't live on.
You need to determine the cost basis of the property you are selling. If you are selling the original house with the original lot, your cost basis probably be the purchase price, minus the value of the land that you subdivided. (The value of the land that you subdivided becomes the cost basis of the second lot where your main residence is now, plus the cost of building that residence.) Then, your capital gains are the difference between the selling price and the cost basis. You can increase the cost basis by certain closing costs and by the cost of any permanent improvements made to the home. There is some guidance here.
https://www.irs.gov/forms-pubs/about-publication-523
https://www.irs.gov/forms-pubs/about-publication-551
You also said the first home became "investment property." Does that mean you rented it? Then your capital gains transaction includes paying depreciation recapture tax on any depreciation you took or could have taken while it was a rental. Also, up until 2017, it was possible to "capitalize" carrying costs on investment property. If you did this, that also adds to the cost basis. If you did not, or you don't know what I mean, don't worry about it because it is too late to change anything now.
If you are selling the lot that contains your primary home you can claim the personal capital gains exclusion of up to $250,000 (or $500,000 for married filing jointly). You can also claim the exclusion if you sell both lots within 2 years of each other (not two separate exclusions, but you can extend one exclusion over both sales). If you only sell the lot you don't live on and keep living in the other house, you can't use the exclusion on the sale of the first house.
You don't submit proof with your tax return, but keep your proof and other documents for 6 years after you sell. If you are audited the IRS does not have to credit any cost basis that you can't prove with adequate reliable records.
