You're going to sell your house: the number to know before you go to market (not at the notary)

The scene no seller forgets: the closing, a ready buyer, and the notary reports that hundreds of thousands of pesos of ISR will be withheld from the price — money no one budgeted. The deal tightens or falls apart — and the alternatives that existed three months earlier are no longer on the table. There is only one professional rule: you ask for the tax calculation before listing the property, not on the day of closing.

First line of defense: the primary-residence exemption

If you sell the home you live in, art. 93, fracc. XIX, inciso a) exempts the gain up to 700,000 UDIS (inflation-indexed units) — roughly $6 million at the current UDI (estimate; the UDI moves daily) — with two requirements: proving to the notary that it is your primary residence (utility bills, ID, tax domicile) and not having used the exemption in the three prior years, which the notary verifies. You sell for $8M what cost $3M: a $5M gain, below the ceiling — zero ISR if you qualify. Only the excess above the ceiling, when it exists, enters the calculation with deductions.

Second line: the four deductions of art. 121

For the taxable gain (a second home, a commercial unit, a lot, or the exemption's excess), the base is not the price — it's the profit after deducting:

• Verified acquisition cost, adjusted for inflation — with the technical nuance that changes results: the construction is reduced by 3% for each year elapsed (never below 20% of the cost) and then adjusted; the land is only adjusted. On old properties, the adjustment plays strongly in your favor.
• Improvements, additions and construction — with their receipts. The $800k remodel with no CFDI is a dead deduction: keep the invoices for every job starting today, even if you don't plan to sell.
• Notary fees, taxes and duties paid on the acquisition (including the ISAI you paid when you bought) and the appraisals.
• Commissions and brokerage — the broker's fee, when buying and when selling, with an invoice.

What can be done with time (and not at the notary)

With months of lead time, moves that no longer fit on the day of closing become possible: gathering or replacing improvement receipts, correctly proving the primary residence (statements in the wrong spouse's name are an avoidable classic), assessing whether it's worth waiting to meet the three-year deadline of the previous exemption, reviewing the co-ownership (each co-owner applies their own exemption and their own rate — sometimes it pays to reorder ownership beforehand, with its own costs), and modeling the accumulation of the gain on the annual return, where the notary's calculation is a provisional payment and your real situation may return money to you. None of these is exotic; all require lead time.