GST at nine per cent, on top of the certified value
GST is not part of what the work is worth. It is a tax added on the invoice at nine per cent, and mixing it into the valuation quietly corrupts every figure downstream.
QScope Team · 14 April 2026 · 5 min read
On a Singapore project the valuation answers one question: what is the work worth to date. GST answers a different one: what tax is charged on that supply. Since the first of January 2024 the standard rate has been nine per cent, and it belongs on top of the certified value, not inside it.
Two numbers, kept apart
The payment claim states the net certified value, the cumulative worth of the works less what has already been claimed. GST at nine per cent is then added on the tax invoice. The total the respondent pays is the net certified sum plus the tax, and both should be visible as separate lines.
- Net certified value, the figure the quantity surveyor values and the SOP Act works from.
- GST at nine per cent, the tax added on the invoice, remitted to the authorities.
- Gross payable, the two added together, the sum the respondent settles.
Why the separation matters
Retention, variations and interim assessments all work on the net value. If GST is folded into the certified figure, retention gets calculated on a taxed number, the running total drifts, and the payment response ends up arguing about a figure that was never the value of the work. Keeping the tax as its own line keeps every other calculation honest.
It also matters for the statutory mechanism. When a payment claim goes unanswered and the matter reaches adjudication under the SOP Act, the adjudicator values the work. GST is a tax consequence of that value, not part of the assessment. A clean net figure is the one that is easy to defend.
The invoice and the claim
Standard forms such as the PSSCOC and SIA conditions certify the value of the works; the GST sits on the tax invoice raised against that certification. Keep the two documents aligned but distinct. The certificate says what the work is worth. The invoice adds the nine per cent and states the gross.
The rule is simple and it holds on every cycle. Value net, retain on net, claim net, and add nine per cent at the end. The certified value is the number that carries all the way through. GST is the last line, never the buried one.
QScope carries GST as a separate line on the tax invoice, keeps the certified value net, and shows both, so the number the SOP Act runs on is never a gross figure in disguise.