Corporation Tax for Agents

Start with HMRC’s Corporation Tax for Agents online service. Use your Agent Services Account (ASA) to manage access, authorise clients and file on their behalf. If you already use HMRC Online Services, link that to your ASA so existing Self Assessment/CT client authorisations flow through. HMRC’s overview and sign-in are here: Corporation Tax for Agents online service.

Tip: Agree with each client who is the appointed agent (HMRC supports one “main” agent and additional “supporting” agents). Keep a single place internally for ASA invites and authorisation status so jobs don’t stall at the first step.

Before you can file, make sure the company is registered for Corporation Tax (within 3 months of starting to trade). Gather and confirm:

• Company name, CRN and registered office
• UTR (and that HMRC has the current RO address)
• Date trade commenced and the accounting period you’ll file for
• Contact details for a responsible officer
• Brief description of the trade

Check HMRC’s record shows the same accounting period dates you intend to file, period mismatches are a frequent cause of bounced submissions.

Every submission needs three aligned parts:

1. CT600 return for the accounting period (maximum 12 months per return).
2. iXBRL accounts (tagged statutory accounts).
3. iXBRL computation (the tax comp that bridges accounts profit to taxable profit/charge).

If the first Companies House period exceeds 12 months, file two CT600s to cover it (e.g., 12 months + the stub period). Attach any supporting PDFs HMRC expects for the claims you’re making (for example, group relief statements), but keep the core accounts and comp in iXBRL.

• Payment of CT: due 9 months and 1 day after the end of the accounting period (unless the company pays by quarterly instalments).
• Filing the return: due 12 months after the end of the accounting period.

Large/very large companies may have to pay by instalments, check HMRC’s guidance for thresholds and timings.

Good practice: Book payment authorisations early (Faster Payments/CHAPS) and avoid weekend/bank-holiday cut-offs. Keep the HMRC payment reference (CT603/statement) tied to your ledger.

Most slow-downs are avoidable. Before you press send:

• Entity & period alignment: Name/CRN/UTR and period dates must match HMRC’s record and be consistent across CT600, accounts and comp.
• iXBRL validity: Run an iXBRL validator; tag primary statements, notes (especially tax), and the computation steps that drive the CT600 boxes.
• Maths that reconciles: CT600 totals must agree to the computation (rounding, signs and add-backs are common culprits).
• Claims with evidence: Loss carry-backs, group relief, donations and non-trading deficits should be supported and consistent period-to-period.
• Close company items: Loans to participators (s455), interest restrictions and associated company counts should be explicitly addressed, not implied.

If HMRC asks for ID or extra evidence (e.g., for a repayment), respond using digital attachments rather than post, decisions are typically faster that way.

• Standardise intake. Use one CT pack checklist (see below) and don’t start computations until it’s complete.
• Work the calendar. Cluster clients by period-end so prep, iXBRL, sign-off, filing and payment reminders run on rails.
• Lock the period early. Confirm final accounts sign-off dates and that no post-year-end journals will change taxable profit after you compute.
• Track amendments. Diary the amendment window (normally 12 months after the filing deadline) and any loss carry-back claim windows.

Ask for this, every time:

• Signed statutory accounts and the final trial balance
• Fixed-asset register, additions/disposals; capital allowances pools
• A schedule of disallowables (client-confirmed: entertainment, fines, etc.)
• Interest paid/received and any CT61 returns
• Losses b/f and elections (group relief, carry-back)
• Associated companies during the period (and dates)
• Loans to participators movements and any repayments post-year-end
• CT payments already made and the repayment bank (for offsets/refunds)

This single page saves three rounds of emails and most late-stage re-works.

Keep the HMRC submission receipt and charge/statement with your working papers, reconcile the CT creditor, and set the payment task. If a repayment is due, ensure bank details on HMRC’s record are current. Where you spot a post-filing adjustment, amend within the allowed window and re-issue payment/refund guidance to the client.