Before you reply: five HMRC letters decoded, and the deadlines that actually matter in 2026

The envelope arrives on a Tuesday, gets put to one side, and acquires a certain weight over the following week. This is the most expensive part of the process. Almost every letter HMRC sends is a notification with a date on it, not a verdict, and the taxpayers who fare best are rarely the ones who know most about tax — they are the ones who read the notice promptly and responded in writing.

Three habits to apply to anything with an HMRC reference on it

First, compare the date of issue with the date you opened it. Most statutory deadlines run from the date printed on the notice, not the day it reached your doormat, which is why a letter left unopened for three weeks can already be halfway through its appeal window.

Second, identify which of four things you are holding: a calculation, an assessment, a penalty, or an enquiry. Each carries a different deadline and a different response.

Third, resist replying the same hour. Read it twice, check the figures against your own records, draft a short factual reply, and send it once. HMRC's systems record every contact; a hurried phone call followed by a contradictory letter creates a muddle that takes months to unpick.

1. The P800 — a calculation, not a demand

A P800 is the output of a PAYE reconciliation. It states that, on HMRC's figures, you paid slightly too much or slightly too little tax through your code. An overpayment is usually repaid automatically. An underpayment of less than £3,000 is normally collected by adjusting next year's code, in which case there is nothing to pay now and the letter is informational.

What to check: the employments and pensions listed, the pay and tax figures for each, and whether anything is missing or duplicated — a second job that ended in April is a common ghost. If the numbers are wrong, say so before the code is amended. There is no penalty attached to a P800, and it is not an accusation of anything.

2. The PA302 — Simple Assessment, and a real bill

Where an underpayment cannot be coded out, HMRC may issue a Simple Assessment. This is a genuine liability, and it is not a Self Assessment return: you cannot discharge it by filing a return for that year. It is payable by 31 January following the end of the tax year concerned, or within three months of issue if that is later.

If you believe the figure is wrong, you can ask for a review within 30 days, setting out precisely which element is disputed and why. If the figure is right but the timing is impossible, ask for Time to Pay before the due date rather than after it. Interest runs regardless; a payment arrangement at least stops the situation widening.

3. The £100 late filing penalty

This one is automatic and arrives even where no tax is owed, which is why it stings. A £100 fixed penalty is issued the day after the filing deadline, followed by daily penalties of £10 (capped at £900) once three months have passed, and further percentage charges at six and twelve months.

You have 30 days from the date of the notice to appeal. The test is reasonable excuse — an unexpected hospital stay, bereavement, a lost records issue that was genuinely unavoidable — and it must be followed by a prompt filing. 'I forgot' and 'my agent was busy' do not qualify. Appeal in writing through the online account, state the facts in order, attach evidence, and file the return regardless. An appeal does not suspend the obligation to file.

4. The compliance check opening letter

This is the one that causes the most anxiety and deserves the least. A compliance check — an enquiry — opens with a letter naming the return or period under review and the aspect being checked. It is a request for information, not a finding. You normally have 30 days to respond in writing.

Read the scope carefully. Is it one figure in one year, or a broad review of several? If the scope looks wider than the stated reason for opening the check, you can ask HMRC to review it. If you use an agent, tell HMRC in writing and let them correspond; if you do not, consider instructing one for this purpose alone.

Send copies, never originals. Answer the question asked, not the question you imagine is coming. Keep a dated log of every item sent and every call made, including the name of the officer and the reference. Silence is the one response that reliably makes a check longer and more expensive than it needed to be.

5. The determination — when a return is missing

Where returns remain outstanding, HMRC can issue a determination: an assessment of tax based on its own estimate. It is not final, and it is not a fine. Filing the return replaces it with a figure based on your actual numbers.

The deadline to appeal is 30 days from the notice. The practical order of events is: file the return, then appeal the determination on the basis that the return has now been submitted. Do both. A determination left unanswered can become enforceable.

When to appeal, when to pay, when to ask for time

Appeal when the facts or the figures are wrong, and do it within the window with evidence attached. Pay when the liability is right, even if the timing is painful — interest and surcharges are avoidable costs. Ask for time when the amount is correct but unaffordable: Time to Pay arrangements are routine, are agreed on the basis of a realistic monthly figure, and are far easier to obtain before the due date than afterwards.

Keeping a clean paper trail

Use the online account where you can, so correspondence is timestamped in one place. Where you write by post, keep a copy and a certificate of posting. Note the date, the reference and the name of anyone you speak to. A tidy file does not change the tax due, but it shortens enquiries, supports appeals and, more often than not, turns a worrying letter back into what it was: a piece of administration with a date on it.