Your balance sheet says the building you operate from is worth $180,000. A commercial broker just told you it would sell for $900,000 tomorrow. Both numbers are correct at the same time — and the gap between them is the reason a lender, a buyer, and the IRS can each look at your books and see a different business. Understanding which number your books show, and why, keeps you from misreading your own financial position when it matters most.
The Cost Principle in One Paragraph
Under U.S. GAAP, nearly every asset on your balance sheet is recorded at its historical cost — what you actually paid for it, including the purchase price plus the costs to get it ready for use, such as delivery, installation, and sales tax. That number then stays put, reduced over time only by depreciation, amortization, or a write-down. It never rises to match the market, no matter how much the asset appreciates. Your books answer the question "what did this cost you?" — never "what could you sell it for today?"
Why Accountants Distrust Market Value
Recording what you paid instead of what something is worth looks strange until you consider the alternative. Historical cost is verifiable: there is an invoice, a canceled check, a closing statement. Two accountants looking at the same purchase will record the same number. Market value, by contrast, is an opinion — one appraiser's $900,000 is another's $750,000, and both change with interest rates, sentiment, and the season.
That subjectivity is exactly what the cost principle guards against. If owners could revalue assets upward whenever they liked, nothing would stop a struggling business from booking a glowing balance sheet by marking up its own property. Lenders know this, which is why loan covenants are written against audited book values rather than management's estimate of market worth. The trade-off is deliberate: your balance sheet sacrifices up-to-date relevance in exchange for numbers nobody can inflate.
This conservatism runs in only one direction. GAAP will happily record bad news — when an asset loses value permanently, you write it down — but it refuses to record good news until you actually sell. That asymmetry has a name, the principle of conservatism, and it explains nearly every exception below: each one exists to recognize losses early, not gains.
How Historical Cost Works in Practice
Suppose you buy a delivery van for $42,000 and pay another $3,000 to outfit it with shelving and signage. The van goes on your books at $45,000 — cost includes everything needed to put the asset into service. If you depreciate it straight-line over five years with no salvage value, each year moves $9,000 from the balance sheet to the income statement as depreciation expense. After three years the books show the van at $18,000, whether used vans of that model currently sell for $25,000 or $8,000.
Land is the purest case: it is never depreciated, so a lot bought in 1998 for $60,000 sits on the books at $60,000 forever, even if the neighborhood around it tripled in value. Buildings, equipment, and furniture follow the same logic minus depreciation. Intangibles you purchase, like a patent or a customer list, are recorded at cost and amortized. The pattern is always the same — start at cost, grind downward on a schedule, ignore the market on the way.
In a Beancount ledger, the same principle shows up as ordinary postings at transaction amounts. You bought the van for $45,000 all-in, so that is what the books carry:
2026-03-14 * "Buy delivery van, outfitted"
Assets:Fixed:Vehicles:Van 45,000.00 USD
Assets:Checking:Business -45,000.00 USD
2026-12-31 * "Year 1 depreciation, delivery van"
Expenses:Depreciation:Vehicles 9,000.00 USD
Assets:Fixed:Vehicles:Van -9,000.00 USDNo entry ever marks the van up to its resale value. If it is totaled or sold, the gain or loss is simply the difference between the cash received and whatever depreciated cost remains — the market only enters your books on the day you actually transact.
The Assets That Break the Rule
Here is where the topic earns its keep. GAAP carves out several asset classes where fair value — what the asset would sell for in an orderly market — overrides historical cost. Every small-business owner should know these, because they are the lines on your statements where market swings show up uninvited.
Marketable securities
Stocks, bonds, and mutual funds you hold as investments are reported at fair value, not cost. Under ASC 320, debt securities classified as trading or available-for-sale are marked to market each period, and equity securities are generally measured at fair value with changes flowing through net income. The logic is practical: unlike your van, these assets have a quoted price every trading day, so fair value is just as verifiable as cost — and far more relevant. If your business holds a $50,000 money-market-plus-equity reserve and the market drops 10%, your income statement takes the hit that quarter even though you sold nothing.
Inventory: lower of cost or market
Inventory follows historical cost until the market falls out from under it. Under the lower-of-cost-or-net-realizable-value rule, you must write inventory down when it can no longer be sold for what it cost you to make or buy — obsolete parts, damaged goods, or products whose selling price collapsed. A retailer holding $30,000 of last season's stock that can now fetch only $18,000 must book a $12,000 loss immediately, in the period the decline happens, not whenever the goods finally sell. Note the one-way ratchet again: if prices recover next quarter, GAAP does not let you write the inventory back up.
Impaired long-lived assets
Equipment, buildings, and other long-lived assets get a reality check when something signals they may be worth less than their book value — a major customer loss, physical damage, a regulation that strands the asset, or persistent operating losses. ASC 360 then requires a two-step impairment test. First, the recoverability screen: compare the asset's carrying amount to the undiscounted cash flows you still expect from using and eventually disposing of it. If the carrying amount is higher, the asset fails. Second, measure the loss as carrying amount minus fair value, and write the asset down to that fair value.
A machine carried at $120,000 that can only generate $90,000 of future cash and would appraise at $70,000 takes a $50,000 impairment loss. The written-down $70,000 becomes the new cost basis for future depreciation, and under U.S. GAAP that write-down is permanent — even if the market rebounds, you never reverse it. This is the exception owners trip over most often, because nothing forces the test on a schedule; you are expected to notice the triggering event yourself.
Crypto assets: the newest fair-value convert
Until recently, bitcoin and similar holdings were treated as indefinite-lived intangible assets: you impaired them when the price fell and never marked them back up, so balance sheets showed only the worst price the asset had ever touched during your holding period. FASB's ASU 2023-08 ended that asymmetry. For fiscal years beginning after December 15, 2024, qualifying crypto assets must be remeasured to fair value every reporting period, with the change — up or down — recognized in net income. If your business holds crypto on its books, expect your reported earnings to move with the market each quarter.
Derivatives and assets held for sale
Two more worth knowing. Derivatives — forward contracts, options, interest-rate swaps — are always carried at fair value, because their entire economic substance is the market move they track; cost is meaningless for a contract that cost nothing to enter. And once you commit to selling a long-lived asset and it meets the held-for-sale criteria, you stop depreciating it and carry it at the lower of its book value or fair value less cost to sell. Both rules share the same spirit: when sale rather than use is the point, the market price is the relevant number.
The Fair Value Hierarchy: Not All Market Values Are Equal
When GAAP does ask for fair value, ASC 820 ranks the inputs by reliability into three levels. Level 1 is a quoted price for the identical asset in an active market — a share of a listed stock, an ounce of gold. Level 2 uses observable inputs other than direct quotes: interest rates, yield curves, prices for similar (not identical) assets, like valuing your building from recent sales of comparable properties nearby. Level 3 is unobservable — your own model, your own assumptions, your own discounted cash-flow projection for an asset nothing like anything that trades.
The hierarchy matters because Level 3 values carry the most uncertainty and draw the most auditor scrutiny. A small business writing down specialized equipment will often land in Level 3, which means documenting your assumptions carefully: the cash-flow forecast, the discount rate, the comparable sales you rejected and why. The number does not have to be perfect, but the reasoning has to be defensible.
GAAP vs. IFRS: The Revaluation Gap
One of the sharpest differences between U.S. GAAP and International Financial Reporting Standards is that IFRS permits what GAAP forbids: under IAS 16, companies may choose the revaluation model for property, plant, and equipment, writing entire classes of assets up to fair value with the increase parked in equity as a revaluation surplus. IFRS also allows reversals of most impairment write-downs when values recover. GAAP allows neither — no upward revaluations, no impairment reversals (with narrow exceptions for certain financial assets).
For a domestic small business this is mostly background knowledge, but it bites in two situations: if you ever present statements to a foreign investor or lender accustomed to IFRS balance sheets, your asset base will look understated by comparison; and if you acquire a company reporting under IFRS, its revalued assets come back down to a cost basis in your consolidated GAAP books.
What This Means for You
Three practical takeaways survive all the theory. First, read your own balance sheet with the bias in mind: the asset side understates what you own whenever property or equipment appreciated, and overstates nothing — which means your true equity cushion is usually larger than the statements show, but you cannot borrow against the difference without an appraisal the lender orders itself. Second, watch for impairment triggers the way you watch for past-due receivables. A machine idled by a lost contract, inventory gathering dust for a year, a rental unit sitting vacant — each is a signal to test and write down before your CPA finds it at year-end, when the adjustment lands in the worst possible period. Third, separate book value from decision value. Whether to sell the building, replace the van, or exit a product line is a fair-value question answered with appraisals and cash-flow forecasts, not with the depreciated cost on your books. The books tell you what you spent; only the market tells you what you have.
Your balance sheet is a history of your spending, not an estimate of your wealth. Once you see it that way, the $180,000 building and the $900,000 broker quote stop contradicting each other — one records what you paid, the other guesses what you would receive, and each is the right number for a different decision.
