An avalúo is a formal property appraisal prepared by an authorized appraiser — a perito valuador. In a purchase it establishes the value the closing taxes are calculated on, and banks require one whenever they finance the deal or set up a bank trust.
In practice
The appraiser signs it, the notario puts it in the file, and its figure is compared against the transaction price and the cadastral value. The highest of the three becomes the base for ISABI, so the avalúo is one of the pieces that decides what you pay in tax on closing day.
Its bigger consequence arrives years later. The value written into your escritura is your cost basis: when you sell, the taxable gain is measured against that number, not against what you remember paying. A low value today means a larger gain — and a larger Mexican income tax bill — tomorrow. The guide to selling taxes walks through the scenarios.
Keep the avalúo, the deed and your construction invoices together. They are the evidence your cross-border accountant will use to defend that basis at sale.
Watch out for
The habit of declaring a value below what was actually paid, to shave the acquisition tax. It cuts both ways: it inflates your future taxable gain, and it means signing a statement before a notario that is not true. The saving today rarely covers the bill later.
Related: ISABI · Predial · Escritura · The investment, in numbers