13.9 Million Homes Could Change Hands as America Ages. Selling Her Paid-Off House Could Still Make Social Security Taxable

Realtor.com, in its generational housing succession analysis released October 5, 2026, projects that aging homeowners could free up 13.9 million homes through 2036. It’s a forecast of homes older owners are expected to leave over the next decade. Some will be sold, and for retirees who do sell, a tax surprise may follow.

Picture Linda, a widow in her 70s who receives Social Security and still lives in the house she and her late husband bought decades ago. The mortgage has been gone for years. She plans to sell and move near her daughter, and she assumes that owing nothing on the house makes the sale a clean, tax-free event. Even so, her paid-off house can pull some of her Social Security benefit into taxable income.

Two Definitions of Income Hiding in One Home Sale

Linda thinks about what the house is worth and what lands in her bank account at closing. The IRS cares about gain, which is a narrower number. IRS Publication 523, the agency’s guide to selling a home, figures gain by subtracting selling expenses from the price, then subtracting the adjusted basis. Adjusted basis starts with what she paid for the home and grows with qualifying improvements like a new roof, an addition or a remodeled kitchen.

Her mortgage has no place in that math. Two identical homes with the same price, costs and basis produce the same gain whether one is paid off or carries a large loan. Paying down the loan built Linda’s equity, while her tax basis was left untouched. Her husband’s death, though, may have changed that. His share of the house generally received a new tax value when he died, and in community-property states, the entire home may receive that treatment. That could leave Linda with a much smaller taxable gain.

Next comes the exclusion. Section 121 of the tax code, which governs the home-sale exclusion, lets a seller who owned and lived in the home as a main residence for at least 2 of the 5 years before the sale exclude up to $250,000 of gain. Qualifying married couples filing jointly can exclude up to $500,000. A surviving spouse who sells within 2 years of the spouse’s death can still use the larger limit, but Linda is well past that window. Her ceiling is $250,000.

What Social Security Ignores Entirely

Start with the good news. Social Security’s retirement earnings test, which can temporarily hold back benefits from people under full retirement age who keep working, counts only wages and self-employment income. A gain on a personal home falls outside both categories.

So Linda’s monthly check remains the same after the sale. Social Security won’t withhold anything or recalculate her benefit, however large the gain turns out to be.

Where the Tax Return Picks Up the Gain

The tax return works differently. Section 86 of the tax code, which sets the rules for taxing Social Security, adds her modified adjusted gross income to one-half of her benefits and compares the total to a base amount. Once she goes above that line, some of her benefits become taxable, up to a ceiling of 85% of benefits.

From here, the excluded and taxable pieces of her gain go separate ways. The portion covered by the exclusion stays out of adjusted gross income, so it never enters the benefit formula. Any gain above the exclusion is a taxable capital gain, and all of it counts toward the income that determines how much of her benefit gets taxed.

That creates a stacking effect. The excess gain is taxed on its own, and it can also drag benefits that were previously untaxed into taxable income for the same year. A retiree who has never owed tax on her Social Security can find a real share of it taxed in the year she sells (we listed this one and eight other quiet retirement tax traps in a free guide here). The formula looks at one year of income at a time, so the effect usually goes away the next year once the sale is behind her.

Records Decide the Number Before the Contract Does

Before Linda signs a sale contract, she should gather her original closing papers, receipts for qualifying improvements and an estimate of selling costs. She also needs records showing what the house was worth when her husband died. Those figures could make a considerable difference in how much of the sale is taxable. Owners who have been in a house for decades lose the most to missing receipts, because projects from the 1990s are easy to forget and hard to reconstruct.

With those numbers in hand, she or a tax preparer can see whether her gain falls safely under the exclusion or spills over it. If it spills over, moving other income, like a large retirement account withdrawal, into a different year can keep the stacking from getting worse.

Paying off the mortgage was a milestone worth celebrating. Her purchase price, the home’s value when her husband died, improvements and closing costs all help determine the tax bill. Digging up an old receipt or appraisal now could save her a nasty surprise next April.

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