joogy06 UK corporation tax (CT600)
For a small Ltd. Rates, limits and the marginal-relief fraction are not repeated here — see
accounting-uk-ltd/references/rates-2026-27.md.
1. Two deadlines, and they are not the same date
| Obligation | When |
|---|---|
| PAY the corporation tax | 9 months + 1 day after the end of the accounting period |
| FILE the CT600 | 12 months after the end of the accounting period |
Payment comes first, and by three months. Filing on time while paying late still attracts interest and penalties. This confusion is routine and expensive — state which one you mean.
Filing route: the joint HMRC/Companies House service closed 31 March 2026. A CT600 now goes through commercial software (or an agent); paper only with a reasonable excuse or in Welsh. This skill prepares the figures; it cannot submit them.
2. The computation — accounting profit is not taxable profit
Profit before tax (per the accounts)
+ Add-backs: disallowable expenditure
− Capital allowances
− Losses brought forward / other reliefs
= Taxable total profits
× Rate (with marginal relief if applicable)
= Corporation tax payable
Never file the accounting profit as the taxable profit. They differ by design, and the computation is the document that shows how — HMRC, the user and any reviewing accountant all read it.
Add-backs a small company actually hits
| Item | Treatment |
|---|---|
| Depreciation | Always added back — replaced by capital allowances |
| Client entertaining | Disallowed (staff entertaining within limits is different) |
| Fines and penalties | Disallowed |
| General/unspecified provisions | Disallowed; specific provisions may be allowed |
| Capital items expensed in error | Added back, then claim capital allowances instead |
| Legal fees on capital transactions | Capital, not revenue |
| Non-business / private proportion | Disallowed |
| Gifts (beyond a small branded limit) | Disallowed |
Depreciation is the one that must never be missed. It appears in every set of accounts and is disallowed in every computation.
3. Capital allowances
Plant and machinery relief replaces depreciation for tax:
- Annual Investment Allowance (AIA) — 100% on qualifying plant and machinery up to the limit.
- Full expensing — 100% first-year relief for companies on qualifying new main-rate plant.
- Writing down allowances — main pool and special rate pool, for what does not qualify above.
- Cars are excluded from AIA and get emissions-based rates.
- Structures and buildings have their own separate allowance.
- April 2027 adds two CT600 boxes for a new 40% first-year allowance.
Check the current limits and rates in the rates reference — they move at fiscal events, and a stale AIA limit produces a wrong return.
4. Marginal relief and associated companies — the small-company trap
Between the lower and upper limits, tax is charged at the main rate then reduced by marginal relief.
The limits are divided by the number of associated companies, and pro-rated for accounting periods shorter than 12 months.
That division is the error worth checking first. A director with two companies has limits halved in each — a company with £60,000 profit and one associate is over the halved £25,000 lower limit and is not on the small profits rate at all. Ask about other companies under common control; do not assume there are none.
5. Losses
- Carried forward against future profits (post-2017 losses are more flexible but restricted at higher levels).
- Carried back one year against the previous period's profits.
- Group relief if part of a group.
Losses are an asset. Track them explicitly, on the return and in the accounts, or they get lost.
6. What a filing package contains
- CT600 — the return form (currently CT600 (2026) Version 3)
- Tax computation — accounting profit to tax payable, showing every adjustment
- Statutory accounts in iXBRL — tagged, from
uk-statutory-accounts
All three go to HMRC together. Companies House gets its own copy of the accounts, separately, on its own deadline.
7. Before it goes anywhere
- Books reconciled and the trial balance agrees (
bookkeeping-double-entry) - Depreciation added back
- Capital additions reviewed for allowances rather than left as expenses
- Associated companies confirmed, limits divided
- Period length checked — short or long periods pro-rate, and a long period needs two returns
- Losses brought forward agreed to last year's return
- Tax charge in the accounts agrees the computation
- A qualified accountant reviews it before submission — particularly a first return, a loss claim, a scheme change, or anything with a disclosure
8. Anti-patterns
- Confusing the payment date with the filing date.
- Filing accounting profit as taxable profit.
- Forgetting to add back depreciation.
- Ignoring associated companies when applying the limits.
- Expensing capital items instead of claiming allowances.
- Applying a rate to a period it does not cover — straddling periods need apportionment.
- Losing brought-forward losses by not tracking them.
- Claiming to have filed. You cannot submit a CT600.