I couldn't prove basis on the parcel I sold because all four shared an account
I hold raw land, long. Four parcels bought across about three years, all in one LLC, nothing built, nothing rented. Carrying costs are small and boring: property taxes, occasional mowing, a survey on one, a fence repair on another, annual state filing fee.
I hired a bookkeeper at $145 a month. Competent, responsive, kept the bank feed clean. She set up a single account called Land and a single account called Land Expenses, and every dollar across all four parcels went into one of those two buckets. I looked at the monthly P&L, saw numbers that were roughly what I expected, and signed off for two and a half years.
Then I sold one parcel. Buyer was fine, closing was fine. My CPA asked for the basis on that parcel specifically: purchase price, closing costs, and whichever carrying costs had been capitalized into it rather than deducted along the way. I could not produce it. The books knew what the entity had spent. They did not know which dirt it went into.
The reconstruction took me eleven evenings with old settlement statements, county tax bills and a bank download, and the bookkeeper charged $960 at her hourly rate to redo the entries. The survey was the worst part, because it covered two adjoining parcels and had never been split, so I had to allocate it after the fact and document why. My CPA also flagged that the capitalize-versus-deduct election on carrying costs for unimproved land is a real decision with real consequences and it had never been made deliberately, it had just happened by default in how things got categorized. That is a licensed professional's call and mine was made by a categorization rule.
What it cost: $960 plus a delayed filing plus eleven evenings I would rather have spent otherwise.
What I'd do differently. Every parcel gets its own subaccount on day one, with the APN in the account name. Every shared cost gets allocated at the time it's paid, in writing, with the basis for the split noted. And the capitalize question gets asked of the CPA before the first tax bill hits the books, not after a sale.