What does the Build UK Company Tax Return (CT600 + iXBRL Accounts) skill do?

Convert statutory accounts and tax computations into the HMRC Company Tax Return package — the CT600 form plus iXBRL accounts (FRC taxonomy) and iXBRL computations (CT computational taxonomy) — covering PCTCT from accounting profit, capital allowances and losses, the main/small-profits rate and marginal relief across financial-year boundaries, R&D and supplementary pages, CT600↔computations↔accounts reconciliation, and a worked example

How do I run the Build UK Company Tax Return (CT600 + iXBRL Accounts) skill?

Copy the skill and paste it into your own Claude or ChatGPT, then provide your source data. FinchContext skills run inside your own AI assistant, so your financial data never leaves your control.

Which region does the Build UK Company Tax Return (CT600 + iXBRL Accounts) skill cover?

United Kingdom. It targets the local tax authority / e-invoicing format described on this page.

Build UK Company Tax Return (CT600 + iXBRL Accounts)

Skill: Convert company accounts & computations into a CT600 Company Tax Return package

Region: United Kingdom Category: Tax — Corporation Tax (CT600) Does: Takes a company's statutory accounts and tax computations and assembles the Company Tax Return package — the CT600 form plus iXBRL accounts (FRC taxonomy) and iXBRL computations (CT computational taxonomy) — for online submission to HMRC Corporation Tax. Spec: HMRC Corporation Tax online — CT600 + Inline XBRL accounts & computations (FRC / CT taxonomies)

A complete CT return is three linked parts: the CT600 (and any supplementary pages), the accounts tagged in iXBRL, and the computations tagged in iXBRL. HMRC requires the accounts and computations as iXBRL attachments. The accounts often also go to Companies House (a joint or separate filing). Box references below follow the CT600, not a verbatim schema.

When this applies

A company within the charge to Corporation Tax files a CT600 for each accounting period; the return is due 12 months after the end of the accounting period (tax is payable 9 months and 1 day after the period end — earlier for large companies via instalments).
The accounting period for tax can differ from a long period of account (which is split into two returns).

Package structure

CT600 form
   Company info (name, UTR, company registration number), accounting period
   Turnover, trading profit, other income, chargeable gains
   Profits chargeable to Corporation Tax (PCTCT)
   Tax calculation (rate, marginal relief), reliefs, R&D, losses
   Tax payable; supplementary pages (CT600A loans to participators, CT600L R&D, etc.)
iXBRL Accounts          statutory accounts tagged with the FRC (UK GAAP/IFRS) taxonomy
iXBRL Computations      tax computations tagged with the CT computational taxonomy

Calculation rules

Compute PCTCT by adjusting accounting profit for tax (add back disallowables, capital allowances instead of depreciation, deduct/relieve losses) — the computations evidence this and are tagged in iXBRL.
Corporation Tax is charged at the applicable rate(s): currently a main rate (25%) with the small profits rate (19%) and marginal relief between the lower/upper limits (limits divided by the number of associated companies) — verify the rates/limits for the financial year(s) the period spans (apportion across FY boundaries).
Reliefs/credits (R&D under the merged/ERIS schemes via CT600L, group relief, losses) reduce the liability; capital allowances (AIA, full expensing) are claimed in the computation.
The three parts must be consistent: CT600 entries reconcile to the iXBRL computations, which reconcile to the iXBRL accounts (turnover, profit).
iXBRL: every tagged fact needs a context (period + entity) and the accounts/computations must validate against the correct taxonomy.

Worked example (outline)

Company UTR ..., CRN 01234567, period 2025-04-01..2026-03-31
Trading profit (accounts) 800,000; add back depreciation 50,000; capital allowances (120,000)
→ PCTCT 730,000
Tax @ 25% main rate = 182,500 (no marginal relief; profit > upper limit)
Less R&D / reliefs as applicable → tax payable
iXBRL accounts (FRC 102 taxonomy) + iXBRL computations attached, reconciling to the CT600

Submitted as the CT600 + iXBRL accounts + iXBRL computations to HMRC Corporation Tax online.

Validation checklist

CT600 company identifiers (UTR, CRN) and accounting period correct; long period of account split into two returns
PCTCT computed from accounting profit via the tax computation (disallowables, capital allowances, losses)
CT rate(s) applied for the financial year(s) the period spans (main/small-profits/marginal relief; associated-company limits); apportioned across FY boundaries
Reliefs/credits claimed on the correct supplementary pages (e.g. CT600L for R&D)
iXBRL accounts (FRC taxonomy) and iXBRL computations (CT taxonomy) attached and valid
CT600 ↔ computations ↔ accounts reconcile (turnover, profit, tax)
Filed within 12 months of period end; tax paid by 9 months + 1 day (instalments if large)

Last updated: 2026-05-31 — CT rates, limits, reliefs (e.g. R&D scheme changes), and taxonomies change each financial year; confirm the current rates/limits, supplementary pages, and iXBRL taxonomy versions against current HMRC Corporation Tax guidance before use.