A C18 post clearance demand note is HMRC’s formal demand for customs duty and import VAT it says was underpaid on an import that has already cleared. It is how HMRC brings additional revenue to account after the original declaration was processed.
Payment is normally due within 10 calendar days of receiving the demand note. After that, interest is charged.
That short deadline is the single most important thing to know, because it is considerably shorter than people expect and it runs whether or not you agree with the demand.
Why you have received one
A C18 follows from one of two routes:
You told HMRC. If you made a voluntary disclosure on a C2001, the C18 is the resulting charge. This is the expected, unalarming version — you already knew it was coming.
HMRC found it. More commonly the C18 follows a compliance check or a post-clearance review. The usual underlying causes are the same handful:
- The wrong commodity code, giving a lower duty rate than applied
- A preference claim that could not be evidenced, so full duty is due
- An understated customs value — freight, insurance, royalties or assists omitted
- A special procedure condition not met, so relief is withdrawn
- Quantity or weight discrepancies
Because HMRC can assess across a four-year window, a C18 rarely covers one entry. If a code was wrong, it was probably wrong every time that product was imported — which is why the figure is often much larger than the shipment that triggered the review.
The first 48 hours
1. Check the deadline on your letter. Do not rely on this article for your date — read what you have been sent. The payment deadline and the response deadline are usually different, and they are both on the paperwork.
2. Work out whether you agree. The demand should identify the entries and the basis. Compare it against your own records: the declarations, invoices, valuation evidence and origin proofs. This is where having evidence organised per MRN stops being theoretical.
3. Do not ignore it. Interest accrues, and HMRC has recovery powers. A demand you dispute is still a demand until the dispute is resolved.
4. Take advice if the sum is significant. Particularly where the assessment spans years or rests on a classification judgement — those are arguable, and the argument is technical.
If you disagree
You have the right to challenge HMRC’s decision. In broad terms there are two routes, and they are not mutually exclusive:
Ask HMRC to review the decision. An internal review by an officer not involved in the original decision. If you provide information and HMRC’s view does not change — or you do not respond within the period stated — you will receive a final decision letter.
Appeal to the independent tax tribunal. Available if you remain in disagreement after review.
Deadlines are stated on your decision letter and they are short. Typically you are given 30 days to respond or request a review from the date of the decision. Check your own letter and act inside the period it states — missing the window is one of the few genuinely irrecoverable mistakes here.
On paying while disputing: paying stops interest accruing, and for indirect tax appeals the position on whether disputed amounts must be paid first has nuance — including the possibility of applying on hardship grounds. If you intend to dispute a significant sum, get advice on this specific point before deciding whether to pay, rather than assuming either way.
The strongest grounds for challenge
Not every C18 is correct. The demands most often reduced or withdrawn are those resting on:
A contested classification. Commodity code decisions are judgements, not facts. If you have a properly reasoned classification supported by product specifications, it is arguable — and worth arguing, because the rate difference applies across every affected entry.
Origin evidence that exists but was not produced. If preference was validly claimed and the proof of origin exists, a demand based on “unevidenced” preference can be answered with the evidence.
Valuation methodology. Whether a particular charge properly forms part of the customs value is a technical question with real room for disagreement.
Arithmetic and scope. Check which entries are actually included and whether the period is right. Assessments are sometimes wider than the error justifies.
What is rarely worth arguing is that you did not know, or that your broker did it. Under direct representation the importer is solely liable for the customs debt — your recourse against an agent is a contractual matter between you and them, not a defence against HMRC.
If the C18 is right
Then the useful questions become forward-looking:
How many other entries does this affect? If HMRC found one instance, the same error almost certainly exists elsewhere. Finding the rest yourself and disclosing them is materially better than waiting for a second demand — although note that once a check is open, disclosure is prompted rather than unprompted, so the penalty reduction is smaller than it would have been.
Has it been fixed going forward? A demand for a past error is much worse if the same error is still being made today.
Is there a corresponding overpayment? The same weak classification process that underpaid on one product often overpaid on another. Those can be reclaimed on a C285 within three years, which sometimes materially offsets the demand.
Avoiding the next one
A C18 is a lagging indicator. By the time it arrives, the error has usually been repeating for years and the assessment covers all of it.
The only real protection is noticing errors yourself, early — which means reconciling what was declared against what you actually bought, continuously rather than annually. You can start with HMRC’s own data, free, going back four years.
The HC Customs compliance portal automates that reconciliation: it pulls your CDS declarations as an authorised third party, checks them against your purchase and goods-received records, flags classification inconsistency and duty anomalies as they arise, and keeps audit-defence evidence assembled per MRN — so if a demand does arrive, you can evaluate it against your own records in hours rather than weeks.
30-day free trial, no card required.
Frequently asked questions
What is a C18 post clearance demand note?
HMRC’s formal demand for customs duty and import VAT underpaid on goods that have already cleared customs. It brings to account revenue found due after the original declaration was processed.
How long do I have to pay a C18?
Normally within 10 calendar days of receiving the demand note, after which interest is charged. Check the date on your own letter.
Can I appeal a C18?
Yes. You can ask HMRC to review the decision, and if you still disagree you can appeal to the independent tax tribunal. The deadlines are stated on your decision letter and are short — typically 30 days — so act within the period given.
Do I have to pay before I can appeal?
Paying stops interest accruing. Whether a disputed amount must be paid before an appeal proceeds has nuance in indirect tax, including hardship provisions. Take advice on this point specifically before deciding.
How far back can a C18 go?
HMRC can generally assess underpaid customs duty across a four-year window, so a demand may cover several years of entries rather than a single shipment.
My customs agent made the mistake — am I still liable?
Under direct representation, yes: the importer is solely liable for the customs debt. Under indirect representation, importer and agent are jointly and severally liable. Any claim against your agent is a separate contractual matter.
What if I cannot pay?
Contact HMRC rather than letting the deadline pass. Time to pay arrangements exist, and engaging early is treated very differently from silence.