Claims are cumulative rather than periodic, retention accrues and releases on its own schedule, and variations enter the claim before they are approved.
Progress claims are one of the few document types where the arithmetic is fully determined and almost never checked in full, because doing so by hand for every subcontractor every month is more work than anyone has time for.
The process
A subcontractor holds a contract for a sum. Each period they claim for work completed to date, expressed cumulatively: total value of work done, less previously certified, less retention, plus approved variations. The main contractor or client certifies an amount and pays it.
Retention is withheld at a contractual rate up to a limit, released partly at practical completion and the remainder at the end of the defects liability period.
Where it breaks
Cumulative claims hide period errors. Because each claim states work to date rather than work this period, an overstatement in one period is carried into every subsequent claim. If the error is never found, it is eventually paid in full at the final account, and the individual claims that carried it all appeared internally consistent.
Previously certified does not match what was actually certified. The claim states a figure for prior certifications. Where that figure is the subcontractor’s record of what they claimed rather than what was certified, the difference is claimed again.
Variations enter before approval. Work instructed but not yet valued appears in the claim at the subcontractor’s assessment. Certifying the claim total without separating approved from unapproved variations approves the valuation by implication.
Retention is calculated on the wrong base or past its limit. Retention applies to a defined base and stops at a cap. Continuing to accrue past the cap, or applying it to amounts it does not cover, produces a persistent small error in the withheld balance that only surfaces at release — by which point the contract is closing and nobody wants to reopen it.
Release is triggered by date rather than by event. The defects liability period runs from practical completion, and practical completion is a certified event, not a planned date. Releasing on the planned date pays retention before the entitlement exists.
The contract sum itself moves. Every approved variation changes it. Validating against the original sum rejects legitimate claims; validating against a sum nobody maintains validates nothing.
How we would build it
Maintain the contract sum as a derived figure, not a stored one: original sum plus the sum of approved variations, each with its approval reference and date. The current sum is then always reconstructible and always explicable.
Rebuild every claim from first principles. Gross value claimed to date, less the amount actually certified previously — taken from the certification record, never from the claim — less retention computed from the schedule, equals the amount now due. Compare against the claimed figure and report the difference.
Separate approved from unapproved variations in the claim, and certify only the approved portion, with the remainder listed rather than silently dropped. A subcontractor whose unapproved work is visibly acknowledged and pending is in a different position from one whose claim was quietly reduced.
Model retention as a running balance with accrual, cap and release events, reconciled every period. The balance is checkable against the sum of all retention deducted to date, which is an independent path to the same number.
Extract the claim’s figures from the submitted document rather than re-keying them, with the document’s own internal arithmetic validated as a condition of acceptance — a claim whose stated components do not sum to its stated total is returned rather than interpreted.
What the system proves about itself
Every certified amount is reproducible from the contract sum, the approved variation register, the certification history and the retention schedule. Nothing depends on a figure typed by hand, so nothing is unverifiable at final account.
The most valuable output is not the certification itself but the register that accumulates behind it: total certified against contract sum, retention held against retention due, unapproved variations outstanding and how long they have been outstanding. That register answers the questions that get asked at the end of a project, at the point where the records are usually least complete and the amounts are largest.