Adjusted basis is the total amount you have invested in a property for tax purposes, and it is the figure the IRS subtracts from your sale price to determine how much of your profit is taxable.1Office of the Law Revision Counsel. 26 USC 1011 – Adjusted Basis for Determining Gain or Loss It starts with what you paid (or another starting value set by law), rises when you put more money into the asset, and falls when you recover part of your investment through deductions or reimbursements. Get it right and you pay tax only on real profit. Get it wrong and you either overpay or underreport.
The concept applies to almost anything you can sell at a gain or loss: real estate, business equipment, stocks, mutual funds, collectibles. The mechanics differ a little by asset type, but the underlying idea does not change.
The Formula
The calculation is straightforward once you know the pieces:
Adjusted Basis = Initial Cost Basis + Capital Improvements and Other Increases − Depreciation, Losses, and Other Decreases
Say you buy a rental duplex for $250,000, including qualifying closing costs. Over eight years, you spend $35,000 on a new roof and updated plumbing, and you claim $55,000 in depreciation. Your adjusted basis is $250,000 + $35,000 − $55,000 = $230,000. Sell for $340,000 and your taxable gain is $110,000, not $340,000.
One limit worth knowing: for most individual taxpayers, adjusted basis cannot drop below zero for purposes of calculating gain. Once depreciation and other reductions have used up your basis, the entire sale proceeds become taxable.
Where Basis Starts
Property You Bought
Your basis generally starts with the cost of the property.2Office of the Law Revision Counsel. 26 USC 1012 – Basis of Property Cost “Cost” is broader than the purchase price. It includes any debt you take on as part of the deal, plus sales tax, freight, and installation for personal property like equipment or vehicles.
For real estate, several closing costs fold into basis: abstract fees, legal fees for the title search and deed preparation, recording fees, transfer taxes, surveys, title insurance, and utility connection charges.3Internal Revenue Service. Publication 551, Basis of Assets If you agree to pay expenses the seller owed, like back taxes, those amounts add to your basis too.
Loan-related costs are excluded. Mortgage points, loan origination fees, mortgage insurance premiums, loan assumption fees, lender-required appraisal fees, and credit report charges do not increase your basis in the property.3Internal Revenue Service. Publication 551, Basis of Assets Casualty insurance premiums and rent paid before closing are also excluded. Some of these may be deductible elsewhere on your return, but none of them raises basis.
Property You Inherited
Inherited property generally gets a new basis equal to its fair market value on the date the previous owner died.4Office of the Law Revision Counsel. 26 USC 1014 – Basis of Property Acquired From a Decedent This is the step-up in basis, and it can erase decades of appreciation in a single moment. If your parent bought a house in 1985 for $80,000 and it was worth $450,000 at death, your basis is $450,000. Sell the next month for $455,000 and your gain is only $5,000. What the decedent originally paid is irrelevant.
If the estate files an estate tax return, the executor may elect an alternate valuation date six months after death, which sometimes yields a lower value.5Internal Revenue Service. Frequently Asked Questions on Gifts and Inheritances
Property You Received as a Gift
Gifts follow a trickier path. Your basis for calculating a future gain is generally the donor’s adjusted basis, called a carryover basis.6Office of the Law Revision Counsel. 26 USC 1015 – Basis of Property Acquired by Gifts and Transfers in Trust If your uncle paid $50,000 for stock and gives it to you when it is worth $80,000, your gain basis is $50,000.
The wrinkle: when fair market value at the time of the gift is lower than the donor’s basis, you use two different figures. For gain, you use the donor’s basis. For loss, you use the lower fair market value at the time of the gift. If you sell for a price between the two, you have neither a gain nor a loss.7eCFR. 26 CFR 1.1015-1 – Basis of Property Acquired by Gift
The IRS regulation gives a clean example: a donor gives property with a $100,000 adjusted basis when fair market value is $90,000. You later sell for $95,000. No gain, because the gain basis is $100,000. No loss, because the loss basis is $90,000. The $95,000 sale sits in the dead zone between them and produces zero taxable result.7eCFR. 26 CFR 1.1015-1 – Basis of Property Acquired by Gift
What Increases Basis
After the starting point, certain expenditures during ownership push basis higher. The main category is capital improvements: expenditures that add value, extend the property’s useful life, or adapt it to a new use. A new roof, a room addition, a full HVAC replacement, or a paved driveway all qualify. Legal fees to defend or perfect title, zoning costs, and utility connection expenses also add to basis.8Office of the Law Revision Counsel. 26 USC 1016 – Adjustments to Basis
Routine maintenance and repairs do not increase basis. Fixing a leaky faucet, patching drywall, or repainting a room keeps property in its existing condition rather than improving it. The improvement-versus-repair distinction is one of the most common audit issues in real estate, and the IRS scrutinizes it closely. The rough test: does the work add value or extend the property’s life, or does it merely keep the property functional?
What Decreases Basis
Basis drops whenever you recover part of your investment through a tax deduction or a reimbursement. Depreciation is the biggest driver. If you use property in a business or hold it for rental income, you deduct part of its cost each year, and every dollar deducted comes off your basis.9Office of the Law Revision Counsel. 26 USC 167 – Depreciation Over the life of a rental property, this can reduce basis by tens of thousands of dollars.
Other common reductions:
- Casualty loss deductions from storms, fires, and similar events, plus any insurance reimbursement for the damage, both reduce basis because they represent a return of your investment.10Internal Revenue Service. Publication 547 – Casualties, Disasters, and Thefts
- Section 179 expensing: business owners who deduct the full cost of qualifying equipment in the year of purchase must reduce basis by the deducted amount.11Office of the Law Revision Counsel. 26 USC 179 – Election to Expense Certain Depreciable Business Assets
- Easement payments received for granting a pipeline or utility right-of-way reduce basis in the affected property.
The logic is consistent: the tax code will not let you benefit twice from the same dollar. If a deduction or reimbursement already recovered part of your investment, basis drops so the recovered amount eventually shows up as gain when you sell.
Situations Where Basis Trips People Up
Your Home
Homeowners spend money on a house for years without thinking about basis. A kitchen remodel, a new furnace, or an addition raises it; a broken window replacement or a drain clog does not. Keep receipts. The IRS will not accept your memory of a renovation from 15 years ago.
When you sell a primary residence, you can exclude up to $250,000 of gain from income, or $500,000 if married filing jointly, provided you owned and lived in the home for at least two of the five years before the sale.12Office of the Law Revision Counsel. 26 USC 121 – Exclusion of Gain From Sale of Principal Residence A higher basis means a smaller gain, which makes it more likely the exclusion covers everything. In high-appreciation markets, every documented improvement matters.
One trap: if you claimed a home office deduction using the actual-expense method, you likely also claimed depreciation on the business-use portion of the home. That depreciation reduces basis, and the depreciated amount may be subject to recapture when you sell even if the rest of the gain qualifies for the exclusion. Taxpayers who used the simplified home office method avoid this because depreciation under that method is treated as zero.13Internal Revenue Service. FAQs – Simplified Method for Home Office Deduction
Stocks and Mutual Funds
One of the most common basis mistakes involves dividend reinvestment plans. When dividends buy additional shares automatically, each reinvestment is a separate purchase with its own basis equal to the amount reinvested.14Internal Revenue Service. Stocks (Options, Splits, Traders) 3 You already paid tax on those dividends as income. Fail to add their cost to basis and you get taxed on the same money twice when you sell. Over a decade of reinvestment, the missed basis can run to thousands of dollars.
Stock splits do not change your total basis; you spread the existing basis across the new share count. A two-for-one split on 100 shares with a $5,000 basis gives you 200 shares at $25 each.
Wash sales work differently. If you sell stock at a loss and buy substantially identical stock within 30 days before or after the sale, the loss is disallowed.15Office of the Law Revision Counsel. 26 USC 1091 – Loss From Wash Sales of Stock or Securities It does not disappear. The disallowed loss is added to the basis of the replacement shares. Sell for a $250 loss and buy replacement shares for $800, and your basis in the new shares becomes $1,050.16Internal Revenue Service. Case Study 1 – Wash Sales
For securities purchased after 2010, brokers are required to track and report your cost basis on Form 1099-B when you sell.17Internal Revenue Service. Instructions for Form 1099-B (2026) For older holdings or securities transferred between brokers without full documentation, tracking is on you.
Like-Kind Exchanges
A Section 1031 exchange lets you swap one piece of investment or business real estate for another and defer tax on any gain. The catch is that basis in the replacement property carries over from the property you gave up, reduced by any cash received.18Office of the Law Revision Counsel. 26 USC 1031 – Exchange of Real Property Held for Productive Use or Investment If the relinquished property had a $150,000 adjusted basis and was worth $400,000, your new property starts with a $150,000 basis. The $250,000 of deferred gain is baked into that lower number and comes due when you eventually sell the replacement in a taxable transaction. This is deferral, not elimination.
Depreciation You Didn’t Take
One point worth flagging: even if you forgot to claim depreciation during the years you owned a rental property, the IRS still reduces basis by the amount you were allowed to deduct. Skipping the deduction does not preserve basis. You get the lower basis whether or not you took the write-off, so there is never an advantage to skipping depreciation on property where you qualify. For real property, the depreciation you claimed or could have claimed is taxed at a maximum rate of 25% when you sell at a gain, separate from the capital gains rate on the rest.
Keeping the Records That Prove Your Basis
You need to keep records supporting your basis for as long as you own the property, plus the years the IRS has to audit the return on which you report the sale. For a rental held 20 years, that means keeping purchase documents, closing statements, improvement receipts, and depreciation schedules for over two decades. If you received the property in a nontaxable exchange, keep records for both the old and the new property until the limitations period runs on the year you finally sell in a taxable transaction.19Internal Revenue Service. How Long Should I Keep Records
The consequences of poor record-keeping are concrete. In a dispute with the IRS, the burden of proof shifts to the government only if you have complied with substantiation requirements and kept all required records.20Office of the Law Revision Counsel. 26 USC 7491 – Burden of Proof Without documentation, you bear the burden of proving the IRS is wrong. Taxpayers who lack records often end up with a basis of zero, meaning the entire sale proceeds are treated as taxable gain.
When you eventually sell, the adjusted basis flows onto Form 8949 and Schedule D, where the sale price minus basis produces the gain or loss you report.21Internal Revenue Service. Instructions for Form 8949 An understated basis inflates your tax bill. An overstated basis creates a phantom loss that can trigger penalties if the IRS catches it. Either way, the number you have been tracking for years is the number that decides what you owe.