When an engineering office complains about cash-flow pressure, the ready explanation is that clients pay late. In many cases that is only partly true.
Because a client doesn't pay before a payment certificate reaches them. A certificate that should have gone out on the first of the month and went out on the twentieth delayed payment by twenty days before the client ever touched it. In most offices, the cash-flow problem starts inside the office, not outside it.
A Payment Certificate Is Not an Invoice
An invoice is a demand for an amount. A payment certificate is evidence that a defined portion of the work has been completed and that payment for it is due under the contract.
That is why a good certificate isn't argued over. It doesn't ask the client to trust you — it shows them what can be verified: which stages are complete, the progress percentage of each, what was previously certified, and how the final figure was reached.
A certificate carrying an amount without that detail invites questions — and every question costs another week.
The Basis of Calculation: Percentage of Completion
Every certificate rests on one question: how much of the contract is complete to date? There are two ways to answer it.
Stage-Based Calculation
The contract is split into stages with pre-agreed weightings — preliminary study, concept design, detailed design, tender documents, supervision. Completing a stage entitles you to its full weighting.
This method is clearer and far less contentious, because the question becomes binary: is the stage complete or not? And that is answered by a delivered document, not by an opinion.
Estimate-Based Calculation
Estimating a percentage of what has been completed inside an ongoing stage. Flexible, but it opens the door to disagreement: you see 70% and the client sees 50%, and neither of you has anything to settle it.
The practical rule: use stages as your basis, and use estimation only inside long stages, tied to defined deliverables rather than to impressions.
What a Certificate Should Contain
| Line item | Note |
|---|---|
| Effective contract value | Original plus approved change orders, not the first figure |
| Work completed to date | Cumulative from project start, not just this period |
| Previously certified | Total certified in earlier certificates |
| Due this period | The difference between the two lines above |
| Advance payment recovery | At the agreed percentage from every certificate |
| Retention deduction | A withheld percentage released per contract terms |
| Tax | Per your country's regulations |
| Net amount due | One clear figure at the end |
The most important line in that table is the first. An office that builds its certificate on the original contract value after three approved change orders claims less than it is owed — then discovers the gap at final settlement, when recovering it is hard.
Advance Payment: How Is It Recovered?
An advance payment isn't revenue — it is funding for the start of the work, recovered gradually by deducting its agreed percentage from every certificate until it is fully repaid.
The common mistake is treating it as immediate revenue, which makes the project look profitable in its first month before cash flow drops for no visible reason. Regular recovery from each certificate keeps the cash picture honest from the start.
Retention: When Is It Released?
A percentage withheld from each certificate and held by the client as security, usually released in two parts: one at preliminary handover, and one at the end of the defects liability period.
It is the most forgotten money in engineering offices — amounts withheld on projects closed two years ago that nobody claims, because nobody remembers them.
The rule: at any moment you should have a live list of retention held and the dates it becomes due for release. That list alone recovers money many offices had written off.
Five Reasons Certificates Go Out Late
- Nobody owns the issuing. The certificate is everyone's job, so it becomes nobody's.
- Progress percentages aren't current. Accounting starts by collecting figures from engineers, burning days before the first line is written.
- Pending change orders. The certificate is held back waiting for one approval, instead of being issued now with the order added next time.
- No fixed date. A certificate issued "when we get a chance" is never issued on time.
- Waiting for the client to ask. The worst of them, because it makes your cash flow depend on somebody else's attention.
Notice that four of the five are entirely internal. That is good news: what is internal can be fixed this month.
What Changes When Certificates Live Inside a System
Inside an engineering consultancy management system, five things change:
- Progress is updated by the stage owner as it happens, rather than collected manually at issuing time
- The effective contract value is already updated by approved change orders
- The certificate is built from existing data — previous, completed, recovery, retention — with no recalculation
- Retention held stays tracked with its release dates instead of being forgotten
- The invoice and the accounting entry are generated from the certificate itself, not from separate data entry
The biggest effect is that issuing turns from a task that takes days into an output that takes minutes — and that alone cuts two weeks out of the collection cycle in most offices.
Frequently Asked Questions
What is the difference between a payment certificate and an invoice?
The certificate proves what has been completed under the contract and calculates the amount due in detail; the invoice is the financial demand issued on the basis of it. In engineering work the certificate always precedes the invoice, because it is the document that gets reviewed and approved.
How is percentage of completion calculated?
Either by stages with weightings agreed in the contract, or by estimating a percentage inside an ongoing stage. The first is clearer and less contentious because it is evidenced by delivered outputs; the second is used inside long stages, tied to defined deliverables.
Is the advance payment deducted from every certificate?
Yes, at the percentage agreed in the contract until it is fully recovered. Treating it as immediate revenue gives a misleading picture of project profitability in its early months.
When is retention released?
Per the contract, usually in two parts: one at preliminary handover and one after the defects liability period ends. More important is keeping a current list of retention held and its dates, because it is the most commonly forgotten money.
What if the client disputes the progress percentage?
Go back to delivered outputs rather than to estimates. If stages are documented with their deliverables and delivery dates, the dispute ends in a record review. If they aren't, the problem lies in the agreed calculation method, not in the client's objection.
Should I issue a certificate while a change order is still unapproved?
Yes — issue it for the currently approved scope and don't hold it back. The change order enters the next certificate once approved. Delaying a whole certificate over one pending item is the most common cause of internal delay.
Conclusion
An engineering office suffers less from weak profit than from profit locked up in work that was completed and never claimed.
Open your live projects and ask: how many have gone more than a month without a certificate despite the work moving forward? That number is your stalled cash — and it is in your hands, not the client's.
Read next: Running an Engineering Consultancy — The Complete Guide From Proposal to Handover.