Running a limited company in London means juggling a long list of obligations, but few carry the financial sting of a missed Corporation Tax deadline. HMRC does not send friendly reminders before penalties land, and the rules around when tax is due, when returns must be filed, and how these dates interact with your accounting period trip up even experienced directors. As tax accountants who work with small and medium-sized businesses across the capital every day, we see the same confusion crop up time and again. This guide sets out exactly what you need to know, in plain English, so your business stays compliant and your cash flow stays predictable.
How Corporation Tax Deadlines Actually Work
Corporation Tax is unusual because the payment deadline and the filing deadline are not the same date. Payment is due nine months and one day after the end of your accounting period, while your Company Tax Return (the CT600) is due twelve months after that same period ends. This gap catches many first-time directors off guard, because it means you must calculate and pay the tax you owe before you have even finished preparing the return that confirms the figure. Understanding this distinction is the first step towards avoiding unnecessary stress and unnecessary charges from HMRC. It also means that a company's actual cash position at the payment date can look very different from its position when the accounts were signed off months earlier, which is exactly why so many London directors find this particular tax more disruptive to plan around than VAT or PAYE.
Why Your Accounting Period Date Matters So Much
Your accounting period is usually set by your company's year end, which is often but not always the same as your financial year for statutory accounts. If your company's first year is longer than twelve months, HMRC splits it into two accounting periods for Corporation Tax purposes, each with its own deadline. This is one of the more technical areas where a specialist tax accountant earns their fee, because getting the split wrong can lead to underpayment, overpayment, or a return submitted against the wrong period entirely. London businesses that grow quickly, change their year end, or go through a restructuring are particularly exposed to this complexity. Groups with multiple subsidiaries face an extra layer of difficulty, since each company within the group may have a different accounting period end, meaning several separate deadlines must be tracked in parallel rather than treated as one combined date, and a missed date in even one entity can undermine an otherwise well-run group structure.
The Real Cost of Missing a Payment Date
Late payment interest starts accruing automatically from the day after your payment deadline, calculated daily until the balance is cleared. Unlike some other taxes, there is no grace period and no formal warning before interest begins. For a growing London business with a healthy tax bill, even a few weeks of lateness can add up to a meaningful sum, and persistent late payment can flag your company for closer HMRC scrutiny in future years. Cash flow planning around this date, rather than treating it as an afterthought once accounts are finalised, is essential for keeping costs down. Many London business owners are surprised to learn that HMRC's interest rate on late Corporation Tax tends to sit noticeably above the Bank of England base rate, which makes an overdue balance an expensive form of unplanned borrowing compared with simply setting funds aside as profits are earned throughout the year.
Penalties for Late Filing You Should Never Risk
Filing your Company Tax Return late triggers an automatic penalty of £100, rising to £200 if you are more than three months late. If your return is six months late, HMRC estimates your Corporation Tax bill and adds a further penalty of ten per cent of the unpaid tax, with another ten per cent if you reach twelve months. Persistent lateness across three consecutive accounting periods increases the flat penalties to £500 and £1,000. These figures apply regardless of company size, which means a small London consultancy faces the same fixed penalties as a much larger business, making early preparation disproportionately valuable for smaller firms. It is worth remembering, too, that these penalties apply on top of, rather than instead of, any late payment interest, so a company that is both late to file and late to pay can face two separate cost streams building simultaneously, which is precisely the scenario a good filing calendar is designed to prevent.
- Payment due: nine months and one day after your accounting period ends
- Filing due: twelve months after your accounting period ends
- Interest on late payment accrues daily from the due date
- Late filing penalties start at £100 and escalate quickly
Planning Ahead Instead of Reacting Under Pressure
The businesses that manage Corporation Tax most smoothly are the ones that treat the nine-month payment window as a planning tool rather than a deadline to fear. Setting aside a percentage of profit throughout the year, reviewing draft figures well before the year end, and scheduling time with your accountant early all reduce the risk of a last-minute scramble. This is particularly important for London companies with seasonal income, where cash available in month one of the accounting period may look very different from cash available when the bill falls due. Proactive tax accountants for small businesses can help build a rolling forecast that keeps this liability visible year-round rather than surfacing only at filing time.
Where Professional Support Makes the Biggest Difference
A specialist tax accountant does more than file your return on time. They review your figures for legitimate reliefs and allowances before the deadline arrives, so you are never paying more Corporation Tax than necessary simply because the calculation was rushed. They also handle communication with HMRC if a query arises, request extensions where genuinely justified, and keep a calendar of every relevant date across your group of companies if you operate more than one entity. For London business owners already stretched across sales, staffing and operations, outsourcing this responsibility to a trusted tax and VAT service frees up time while reducing risk considerably.
Common Mistakes We See Among London Companies
The most frequent error is assuming the filing deadline and payment deadline are identical, leading to a return submitted on time but a payment made three months too late, with interest already accruing. Another common mistake is forgetting that a change of year end resets the accounting period calculation, sometimes creating two shorter periods that each need separate treatment. We also regularly see dormant companies incorrectly assume no filing is required at all, when in fact a nil return or confirmation of dormant status may still be necessary. Working with an experienced adviser through our tax and VAT services helps London businesses sidestep these pitfalls before they become expensive. We also frequently encounter directors who assume that appointing an accountant shortly before the deadline is enough to guarantee accuracy, when in reality the strongest results come from a relationship that reviews management accounts throughout the year, well ahead of the return being due.
The Bottom Line
Corporation Tax deadlines are unforgiving, but they are entirely predictable once you understand how they are calculated. Knowing your accounting period, marking both the payment and filing dates separately, and building tax into your cash flow planning throughout the year will keep your London business firmly on the right side of HMRC. If deadlines have crept up on you before, or you simply want the certainty of knowing everything is handled correctly, speaking to a dedicated tax accountant is one of the most cost-effective decisions a growing business can make.
Need Help With Tax?
Let's get your tax right, and keep it that way
Fred Michael & Co Ltd provides tax services for London businesses, freelancers and landlords, from self-assessment to VAT and corporate tax.