The content that makes this subject actionable is the set of tolerance rules, because they determine which numbers on the estimate are promises and which are guesses. Regulation Z sorts every charge into three treatments.
Zero tolerance is the general rule: an estimate is made in good faith only if the charge you eventually pay does not exceed the amount originally disclosed. This is the default, and it captures the lender's own origination charges, transfer taxes, and the cost of any service the lender required but did not permit you to shop for, such as an appraisal it ordered itself. For these items the estimate is effectively binding.
Ten percent, in aggregate applies to a third-party service or a recording fee, and it comes with conditions worth reading rather than summarising. The estimate is in good faith if the aggregate of such charges does not exceed the aggregate disclosed by more than 10 percent, if the charge for the third-party service is not paid to the lender or one of its affiliates, and if the lender permitted you to shop for that service. Two features of this bucket surprise people. It is a test on the group, not on each line, so one figure may rise a great deal provided the bucket as a whole stays within 10 percent. And the conditions matter: a service the lender let you shop for but which you bought from the lender's affiliate does not get the 10 percent cushion.
No limit applies to a defined list, subject only to a requirement that the estimate be based on the best information reasonably available. That list is prepaid interest, property insurance premiums, amounts placed into an escrow or similar account, charges paid to a provider you selected who was not on the lender's written list, and property taxes. Separately, a lender may issue a revised estimate when a changed circumstance defined by the regulation occurs, which resets the baseline the tolerances are measured against.
Laid out that way, the structure is more reassuring than the folklore suggests, and the reason is worth naming: the categories that can move freely are largely the ones that were never the lender's to control, while the categories the lender does control cannot move at all. A prepaid-interest figure changes because your closing date moved. An escrow deposit changes because a tax bill or an insurance quote is what it is. An origination charge, by contrast, is the lender's own number, and it is held to what was disclosed.
Two deadlines make the comparison possible. The Loan Estimate must be delivered or placed in the mail no later than the third business day after the lender receives your application. The Closing Disclosure must be received by you no later than three business days before consummation, the point at which you become contractually obligated on the loan. That second window is the one to use. It exists so that the final figures can be checked against the estimate while there is still time to ask, and the tolerance rules above are what make the checking worthwhile rather than merely informative.
On the size of the bucket, the honest answer is that it varies more by geography than by lender, because two of its largest components are creatures of state and local law: transfer taxes are imposed by states, counties and sometimes municipalities at rates that differ enormously or are not imposed at all, and title practice, including who orders the search and whether an attorney is involved, is also set locally. Any single national figure is therefore an average across places that do not resemble each other.