September Compliance Checklist for Accountants: Your Guide to Staying Ahead
September is here; the kids are back at school, the leaves are just starting to turn, and for accountants across the UK, it means a new batch of tax and reporting deadlines are lining up like buses (and often arriving all at once). Miss one, and you risk HMRC penalties, client panic, and a flurry of “urgent” emails at 4:59 pm on a Friday.
To help you stay on track (and keep your clients stress-free), we’ve compiled this September compliance checklist with key HMRC dates and some practical tips to keep the month running smoothly.
-
VAT Returns – 7 September 2025
What’s due?
VAT returns and payments for the quarter ending 31 July 2025 (plus any monthly filers with additional deadlines).
What to watch out for:
HMRC is all about electronic payments now – cheques are a thing of the past (like floppy disks and dial-up tones). Make sure your clients know that cleared funds are what count, not “we sent it yesterday”.
Supportive tip:
Encourage your clients to submit and pay early where possible. Not only does this avoid the mad scramble, but it gives them time to sort out any surprises before the deadline looms. A gentle reminder email now can save a frantic phone call later.
-
PAYE & NIC (Including CIS Deductions) – 22 September 2025 (19th if by post)
What’s due?
PAYE, Class 1 NIC, and Construction Industry Scheme deductions for the period ending 5 September 2025.
What to watch out for:
The electronic payment deadline is 22 September, but for the handful of clients still sending things by post (yes, they exist), the cut-off is 19 September.
Supportive tip:
Remind clients to set up Direct Debits or schedule payments in advance. Weekends and bank holidays have a habit of sneaking up, and HMRC doesn’t do “oops, it was the weekend” extensions.
-
Corporation Tax – 30 September 2025
What’s due?
Corporation Tax payments for companies with year-ends of 31 December 2024.
What to watch out for:
BACS, CHAPS, online banking – all fine, but processing times vary. Leaving it to the last minute is a recipe for unnecessary penalties (and accountant-induced eye rolls).
Supportive tip:
Get ahead of the game: send your clients a friendly nudge early in the month to allow plenty of time for funds to clear. Late payment penalties on Corporation Tax can escalate faster than a client saying, “It’s just a quick question…”
-
Self-Assessment Payments on Account – Rolling Checks
What’s due?
For any clients who missed the 31 July 2025 deadline for their second payment on account, late payment interest is already ticking away.
What to watch out for:
HMRC won’t send them a polite reminder (you’re the polite reminder).
Supportive tip:
Use September as a checkpoint month: follow up, make a plan, and help them avoid surcharges or debt collection headaches down the line. Sometimes, a simple payment plan conversation now can prevent a much louder conversation later.
-
Advisory Fuel Rates – Effective 1 September 2025
What’s new?
HMRC’s updated advisory fuel rates for company car drivers are now in effect.
What to watch out for:
Mileage claims, expense systems, and policies may all need a quick refresh to ensure compliance.
Supportive tip:
Advise your clients to review their internal policies and update expense templates. A five-minute tweak now beats a month of correcting expense claims after the fact.
Planning Makes Perfect
September tends to be a busy month in practice life – but with the right planning, you can keep your clients compliant, penalty-free, and happy. Proactive reminders, early submissions, and a well-timed phone call can save everyone a headache (and you a weekend of unnecessary stress).
At Capium, we’re here to help you streamline the admin and keep deadlines firmly under control. Our digital tools make VAT, payroll, and Corporation Tax submissions simpler and faster – leaving you more time to grow your practice (or at least enjoy that cup of tea before it goes cold).
Try Capium free today or give us a call on 020 3322 5578 to see how we can help you stay ahead this September.
