Progress billing: the details
It is prepared by measuring what has actually been completed of each line item in the estimate. In Spain the usual practice is to bill cumulatively: everything done since the start is valued and what was billed in earlier months is subtracted, which corrects past errors on its own. The project’s supervising architect or engineer reviews it and, once approved, the contractor invoices. A retention is usually withheld as a guarantee until the job ends.
The work lies in the measurements and in both sides accepting them. If they are kept in different spreadsheets, the argument starts over every month. With measurements taken on site against the estimate’s line items, with their photos, the billing becomes a calculation and the conversation focuses on what is genuinely in doubt.