Ask most investors what a 1031 exchange requires and they will tell you the cash part: roll all your proceeds into the replacement and you defer the tax. True, but incomplete, and the missing half ambushes people at the closing table. To defer all of your gain, you generally have to replace not just your equity but your debt. Pay off a large mortgage when you sell and buy the replacement with a much smaller one, and the difference is treated as if you pocketed cash. That phantom cash has a name, mortgage boot, and it is taxable even though not a dollar ever hit your bank account.
Two things have to go up, not just one
Picture your sale as two buckets: the equity you walk away with, and the debt that gets paid off. A full 1031 deferral asks you to reinvest the first and replace the second. Concretely, to defer everything you generally need to buy a replacement of equal or greater value, reinvest all of your net equity, and take on debt equal to or greater than the debt you retired. Fall short on the equity and you have cash boot. Fall short on the debt and you have mortgage boot. Either shortfall is taxable, up to the amount of your gain.
The debt piece is the one people forget, because it feels counterintuitive. Why would the IRS tax you for having less debt? Because in an exchange, being relieved of a mortgage is economically the same as receiving cash: someone else, the buyer, took over that obligation, freeing you of it. If you do not shoulder a comparable obligation on the new property, the tax code treats that relief as value you received.
The math, worked out
Say you sell a building for $1,000,000 with a $400,000 mortgage. After paying off the loan you have $600,000 of equity to reinvest.
- Full deferral: you buy a replacement for at least $1,000,000, put all $600,000 of equity in, and finance at least $400,000. Debt replaced, equity reinvested, gain fully deferred.
- Mortgage boot: you buy a $1,000,000 replacement but only borrow $300,000 and, say, add nothing extra. You are $100,000 short on debt. That $100,000 is mortgage boot, and it is taxable even though you never touched cash.
The trap is quietly buying with less leverage than you had before. A lot of exchangers, sitting on appreciated property, instinctively want to de-lever. Do it inside an exchange without planning and you convert a clean deferral into a partial tax bill.
The two clean ways to cure it
The good news is that mortgage boot is one of the easier problems to solve, and you have two levers:
- Replace the debt with new debt. Finance the replacement with a loan equal to or greater than the one you paid off. Simple, and the most common fix.
- Replace the debt with your own cash. This is the rule people find surprising and useful: you are allowed to make up a debt shortfall by adding fresh cash out of pocket. Cash offsets missing debt. In the example above, wiring an extra $100,000 of your own money into the purchase erases the $100,000 of mortgage boot.
One direction that does not work is the reverse: you cannot offset cash you pulled out by piling on extra debt. Taking on a bigger loan does not let you walk away with tax-free proceeds. Cash cures a debt gap; new debt does not cure a cash grab. Keep that asymmetry straight and most boot problems disappear.
If you genuinely want less leverage on the replacement, a Delaware Statutory Trust can help, because many DST offerings come with built-in, non-recourse debt at the trust level, letting you satisfy your debt-replacement number without personally qualifying for or guaranteeing a new loan. DSTs are offered to accredited investors through private placement and carry real risks, including illiquidity, fees, and loss of principal, so they are a planning option, not a default. And if what you actually want is cash out of the deal, remember you can defer fully and refinance afterward, or deliberately accept boot through a partial exchange with eyes open.
The bottom line
Full 1031 deferral is a two-part test: replace your equity and replace your debt. Mortgage boot is what happens when you clear the first hurdle and trip on the second, and it surprises people precisely because it taxes a benefit, lower debt, that feels like a good thing. The fix is almost always straightforward once you see it coming: match your old debt with new debt, or plug the gap with cash. The one thing you cannot do is improvise it at closing, which is why we map the debt and equity math before you ever sign, as part of our exchange process. You can sketch the numbers yourself first with our calculator.
Frequently asked questions about mortgage boot
Do I have to replace my mortgage in a 1031 exchange?
To defer all of your gain, generally yes. You need debt on the replacement property equal to or greater than the debt paid off on the property you sold, unless you make up the difference with additional cash. Any un-replaced debt is taxable mortgage boot.
What is mortgage boot?
Mortgage boot is the taxable amount created when the debt on your replacement property is less than the debt you were relieved of on the property you sold. Debt relief is treated like cash received, so the shortfall is taxed up to the amount of your gain.
Can I avoid mortgage boot by adding cash?
Yes. You may offset a debt shortfall by contributing additional cash out of pocket. Cash offsets missing debt. The reverse is not true, though: taking on extra debt does not offset cash you pulled out of the exchange.
Why is having less debt taxable?
Because being relieved of a mortgage is economically similar to receiving cash. If the buyer assumes or pays off your loan and you do not take on comparable debt, the IRS treats that relief as value you received.
Can a DST help me meet the debt requirement?
Often, yes. Many DST offerings include built-in non-recourse debt at the trust level, which can satisfy your debt-replacement figure without you personally qualifying for a new loan. DSTs carry their own risks and suit accredited investors, so weigh them with your advisors.