# Uk Corporation Tax > Use when preparing or checking a UK Company Tax Return — the CT600, the tax computation that turns accounting profit into taxable profit, add-backs and disallowables, capital allowances including the annual investment allowance and full expensing, losses, associated companies and marginal relief, and the separate payment and filing deadlines. Covers what a small Ltd must hand to filing software. - Skill: `joogy06/uk-corporation-tax` (Agent Skill) - Install (CLI): `npx skillmds@latest add joogy06/uk-corporation-tax` - Raw SKILL.md: https://api.skillmd.com/api/skills/joogy06/uk-corporation-tax/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Finance & Business - Author: joogy06 (https://skillmd.com/u/joogy06) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/joogy06/uk-corporation-tax --- # 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 1. **CT600** — the return form (currently CT600 (2026) Version 3) 2. **Tax computation** — accounting profit to tax payable, showing every adjustment 3. **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.