Corporation tax deadlines: a year-end tracker for practices

What are the corporation tax deadlines for a limited company?

A UK limited company with taxable profits of up to £1.5 million must pay corporation tax 9 months and 1 day after the end of its accounting period, and file its Company Tax Return (CT600) 12 months after that same date. Private company accounts go to Companies House 9 months after the financial year end. Larger companies pay in quarterly instalments instead.

Those three dates are simple for one company. The trouble starts when a practice looks after 60 or 200 of them. Most of the guides on page one of Google list the dates for a single business. What they skip is how a firm should plan a whole book of companies, and the uncomfortable fact that the money is due three months before the return. If your internal deadline is the CT600 date, you are already late for the payment.

This guide treats the year end as the starting gun for a fixed sequence of work, and shows how to plot every client on one tracker so nothing slides into the January Self Assessment rush.

Which date comes first: payment, accounts or the CT600?

For a typical private company with a 12-month period, the order is fixed. Companies House accounts and the corporation tax payment fall together around the nine-month mark, and the CT600 follows three months later. GOV.UK sets the rules out on its accounts and tax returns for private limited companies page.

Notice what the table implies. To pay the right amount on time, you need a tax computation, which needs finished accounts, which need records and director approval. In practice the payment date, not the filing date, is the one that drives the whole job.

The confirmation statement runs on its own anniversary, set by incorporation date rather than year end. It still belongs on the same tracker, because Companies House will not accept it until every director has verified their identity, and that can take a client longer than you expect.

How do quarterly instalment payments change the timetable?

A company whose taxable profits exceed £1.5 million is treated as large and pays its liability in four equal instalments. HMRC's instalment guidance gives a calendar-year example: payments on 14 July and 14 October during the period, then 14 January and 14 April after it ends. Above £20 million, a very large company pays even earlier, on the 14th day of months 3, 6, 9 and 12 of the period itself.

Two details catch practices out. First, the £1.5 million threshold is shared: it is divided by the number of associated companies plus one, so a client with five associated companies faces a limit of £250,000 each. Second, there are reliefs. No instalments are needed if the liability is under £10,000, or broadly if the company was not large in the previous 12 months and its profits are no more than £10 million.

For these clients the year end is no longer the start of the job. You need a forecast of profit during the year, which means management accounts and a planned conversation with the finance director in month five or six.

What happens if a company misses a deadline?

Late filing and late payment are charged separately, and a company can face both. For returns with a filing date on or after 1 April 2026, HMRC doubled the fixed penalties. According to the GOV.UK late filing penalty page, a CT600 one day late now costs £200, with a further £200 at three months. At six months HMRC estimates the bill and adds 10% of the unpaid tax, and another 10% follows at twelve months. A third late return in a row turns each £200 into £1,000.

Companies House charges its own penalty for late accounts, on a sliding scale for private companies: £150 up to one month late, £375 up to three months, £750 up to six months and £1,500 beyond that.

Late payment attracts interest from the day after the due date. HMRC's published interest rates put corporation tax late payment interest at 7.75% at the time of writing, rising to 8% from 24 November 2026. The rate is the Bank of England base rate plus 4%.

One point worth repeating to clients: the filing penalty applies even when no tax is due. A dormant or loss-making company still has to file.

How should a practice map its clients by year end?

Start with a single export of every limited company client, showing three fields: accounting reference date, company number and whether the client was large last year. Then group the book into year-end cohorts, one per month. Every company in a cohort shares the same three statutory dates, so you plan the cohort, not 40 separate jobs.

Most practices find the book is lumpy. A big March cohort is common, because many companies were set up to match the tax year. December is often the second largest. Those two groups land in your busiest periods: the March companies have a payment date of 1 January, right in the middle of the Self Assessment peak, and the December companies collide with the spring VAT quarter.

Once you can see the lumps, you have choices. You can pull some March work forward into the summer, move a few staff hours across, or, for a client who would genuinely benefit, discuss changing the accounting reference date. A change of year end has its own Companies House rules, so treat it as advice to a single client, never a way to smooth your workload.

What internal milestones should sit behind each deadline?

Work backwards from the payment date, not forwards from the year end. A sequence that holds up for most small companies looks like this:

1. Year end + 1 day: send the records request, tailored to the client's last-year file.
2. Year end + 8 weeks: records due. Chase at weeks 4, 6 and 8.
3. Year end + 5 months: draft accounts and tax computation ready for review.
4. Year end + 6 months: accounts sent to directors for approval. Our guide to getting directors to sign off on time covers that step in depth.
5. Year end + 7.5 months: the client has the tax figure, payment reference and due date in writing.
6. Year end + 8 months: file accounts at Companies House and the CT600 together, well before either deadline.

Filing the return early has a real advantage. HMRC will not chase a liability it has not been told about, but once the return is in, the client sees the same number in their HMRC account that you gave them, which removes most of the "is this right?" calls.

The one milestone practices most often drop is step 5. A client who learns the figure a week before the due date cannot always find the cash. Six weeks' notice turns a crisis into a bank transfer.

Worked example: a 60-company book on one tracker

Take an illustrative practice with 60 limited company clients. Its year ends split as follows: 22 at 31 March, 12 at 31 December, 9 at 30 June, 8 at 30 September and 9 spread across the other months. One client, a wholesaler, made profits of about £2 million last year and pays by instalments.

Here is how the four main cohorts sit for year ends in 2026:

Now the capacity sum. If each small company takes about 8 hours of preparation and review, the March cohort alone is 176 hours, and the practice wants all of it finished before December. Spread over June to October, that is roughly 35 hours a month. Leave it until the autumn and the same 176 hours land in eight weeks alongside Self Assessment.

The wholesaler sits outside the table. On a 31 March 2026 year end, its first two instalments fell due on 14 October 2025 and 14 January 2026, before the year had even closed, so its work started with a profit forecast in the summer of 2025. Put it on its own line with its own dates.

How do you chase records and approvals without losing weeks?

Three habits make the biggest difference. First, send a specific request, built from last year's file: the bank accounts, loan statements and director's loan movements that apply to that company, not a generic list of 40 items. Second, give the client one place to upload, so files do not arrive scattered across email threads and messaging apps. Third, set the chase sequence when you send the request, so the reminders go out whether or not anyone remembers.

For approvals, send the accounts with a short covering note: the profit, the tax, the payment date and what you need from the director. An approval request with no figures in the body tends to sit unopened.

Keep a simple decision rule for clients who go quiet. The same thinking appears in our quarterly VAT workflow:

Looking further ahead, accounts filing at Companies House is moving to software only. Our note on Companies House software filing from 2028 explains the timing.

How Zaquil handles corporation tax deadlines across a client book

In Zaquil, each limited company client gets a recurring job built from a year-end template, so the moment one year's accounts are filed, next year's job is waiting with its checklist and dates. The firm sees every company on a kanban board by stage (records requested, in preparation, with directors, ready to file), filtered by year-end month, which is the cohort tracker described above.

On the day after the year end, an automation sends the records request. The client opens their branded secure portal, sees exactly which bank statements and loan documents are outstanding, and uploads them against each item. If nothing arrives, the chasers at weeks four, six and eight go out on their own, and the job moves stage when the last item lands.

When the draft is ready, the accounts and tax computation go back through the portal for the directors to approve and e-sign, with a signed PDF certificate kept on file. The firm's deadlines view shows the payment date, the Companies House date and the CT600 date for every company, so a partner can see in one screen which March clients are still waiting for approval in October.

Fees follow the same flow: the year-end invoice can be sent from the job, paid online in the portal, and the filed accounts can be held until it is settled if that is your policy. See the accounting and tax setup for the full workflow, or check Zaquil pricing and start a 14-day free trial with no card.