They erode gradually, through small change orders and "while we're at it" additions that each seem reasonable in isolation but add up to a significantly different total than what was agreed.
Three habits protect against this more reliably than any spreadsheet:
Get the budget broken down by phase before work starts, not as a single lump sum. A phase-by-phase budget makes it obvious the moment something starts running over, rather than surfacing as a surprise total at the end.
Insist that change orders are priced and approved in writing before the work happens, not after. Verbal "it'll just cost a bit more" conversations are where budgets quietly drift. A written change order, even a short one, creates a decision point instead of letting cost creep happen by default.
Tie payment releases to verified phase completion, not to elapsed time or contractor requests. Paying on a schedule that isn't tied to confirmed, inspected progress removes your only real leverage if something isn't done to standard.
The owners who stay in control of a remote renovation budget aren't the ones who check in the most often — constant check-ins without structure just create noise. They're the ones who set up the phase-by-phase structure and the independent verification step before construction starts, so the budget can only move within a structure they've already approved.
If you're heading into a renovation you'll be managing from abroad, the budget conversation that matters most happens before the contract is signed — not during weekly calls once work is underway.