Cost basis significantly affects taxes by determining your capital gains or losses when you sell an asset; the higher your cost basis, the lower your taxable profit (gain), while a lower basis increases the gain, and a loss can offset other gains, reducing your overall tax liability. It's your original purchase price plus fees, used as the baseline to calculate profit or loss, which is then taxed as capital gains, with different rates for short-term (held under a year) and long-term (held over a year) investments.
Cost basis matters because it's the starting point for any calculation of a capital gain or loss. If you sell an investment for more than its cost basis, you'll have a capital gain. If you sell it for less, it's a capital loss. Calculating your cost basis is generally pretty straightforward, but there are exceptions.
Tax basis is the value of an asset used to calculate taxable gain when the asset is sold, transferred, or exchanged. It typically includes the purchase price plus related costs such as taxes, fees, and transportation.
This information is usually provided on a confirmation statement sent to you by your brokerage firm after you purchase a security. You're responsible for reporting your cost basis information accurately to the IRS, in most cases by filling out Form 8949.
If so, you can reduce the taxable gain by including the improvements in the cost basis of the house. If you operate a business from your home or rent a portion of your home to someone, you may be able to write off part of your home's adjusted cost basis through depreciation each year that you use it for that purpose.
On a $100,000 capital gain, you'll likely pay 15% for long-term gains, resulting in about $15,000 in federal tax (plus potential state tax), but it could be 0% or 20% depending on your total taxable income and filing status, while short-term gains are taxed as ordinary income (potentially 22-24%).
Tax deductions reduce the amount of income that is subject to income tax. Some expenses paid during the year, like mortgage interest, can be deducted as itemized deductions. Charitable contributions and medical expenses can also be included as itemized deductions.
Your basis is essentially your investment in an asset—the amount you will use to determine your profit or loss when you sell it. The higher your basis, the less gain there is to be taxed—and therefore, the lower your tax bill. This is why it's so important to accurately track the basis of any investment you own.
Proving Your Cost Basis
Homeowners should keep good records of improvements they have made to a house, including keeping copies of all receipts and purchase orders. If a joint owner of property dies, you should get the property appraised to show the value at the time it is stepped up in basis.
The 20% rule for capital gains refers to the highest federal tax rate for long-term capital gains, applying to higher income brackets when you sell investments (stocks, real estate) held for over a year, with lower rates of 0% and 15% for lower incomes, and even higher rates for special assets like collectibles. This rate kicks in for single filers earning over approximately $492,300 (2024) or $533,401 (2025), and higher for joint filers, making holding assets over a year a key tax strategy.
The IRS 7-year rule primarily applies to keeping records for claiming a deduction for bad debts or losses from worthless securities, allowing a longer period to file for a credit or refund, but it's not a universal audit limit; it's often a recommended safe buffer for general record-keeping, with the standard IRS audit period usually being 3 years, extending to 6 years for substantial income omission (over 25%) or foreign income issues, and indefinitely for fraud.
The "6-year rule" for Capital Gains Tax (CGT) in Australia allows you to treat a former main residence as tax-exempt for up to six years after you move out, even if you rent it out, enabling you to avoid CGT on any growth during that period. You qualify by moving out, choosing to treat it as your main home for tax, and can reset the rule by moving back in. If you rent it out for longer than six years, only the portion of the gain after the six-year mark becomes taxable.
When you sell your primary residence, $250,000 of capital gains (or $500,000 for a couple) are exempted from capital gains taxation. This is generally true only if you have owned and used your home as your main residence for at least two out of the five years prior to the sale.
Not reporting all of your income is an easy-to-avoid red flag that can lead to an audit. Taking excessive business tax deductions and mixing business and personal expenses can lead to an audit. The IRS mostly audits tax returns of those earning more than $200,000 and corporations with more than $10 million in assets.
To prove the IRS's 2-out-of-5-year rule, you must show you owned and lived in your home as your primary residence for at least 24 months (two years) (not necessarily consecutive) within the five years before the sale, using documentation like utility bills, driver's license, voter registration, tax returns, bank statements, and mail all showing the home address. This proves you meet both the ownership and use tests for excluding capital gains on the sale, requiring documentation to back up your claim of residency during that period.
Qualifying for the exclusion
In general, to qualify for the Section 121 exclusion, you must meet both the ownership test and the use test. You're eligible for the exclusion if you have owned and used your home as your main home for a period aggregating at least two years out of the five years prior to its date of sale.