Accountant Guide: Migrating Clients from HMRC's CT600 Service
If you have clients who've been self-filing through HMRC's Company Accounts and Tax Online (CATO) service, they'll need to migrate to commercial software before 31 March 2026. This guide helps you support that transition — or bring them on as new clients.
Understanding Your Clients' Situation
Who Uses HMRC's Free Service?
Clients currently using CATO typically:
- Director-managed micro-entity companies
- Cost-conscious sole director businesses
- Companies with simple affairs (trading, no subsidiaries)
- Those who prefer DIY over professional fees
Why They're Now Looking for Help
The service closure creates uncertainty:
- "What software should I use?"
- "Will it cost me more now?"
- "Is this a good time to get an accountant?"
- "How do I access my old filings?"
Client Communication Strategy
Proactive Outreach
Contact clients before they hear about the closure elsewhere:
Email template: ``` Subject: Important: HMRC CT600 Filing Service Closing
Dear [Client],
You may have heard that HMRC's free CT600 filing service is closing on 31 March 2026. From 1 April 2026, all Corporation Tax returns must be filed using commercial software.
I wanted to reach out to ensure you're prepared for this change.
[Option A - If you want to bring them on as clients] This might be a good time to review whether professional support would benefit your company. I'd be happy to discuss how we can help.
[Option B - If you want to recommend software] There are good software options available, including free ones like TinyTax for micro-entity companies. I'd be happy to advise on what would suit your situation.
Would you like to arrange a call to discuss?
Best regards, [Your name] ```
Key Messages for Clients
- Don't panic — Free options still exist
- Act early — Avoid last-minute rush
- Download history — They'll lose access to HMRC records
- Consider professional help — Complexity may warrant it now
Migration Options to Offer
Option 1: Full Service — Client Joins Your Practice
Best for:
- Clients whose affairs have grown complex
- Those who want to stop DIY filing
- Companies where you already do other work
- CT600 preparation and filing
- Statutory accounts
- Year-end advice
- Tax planning opportunities
Option 2: Supported Self-Filing — Software Recommendation
Best for:
- Truly simple affairs
- Price-sensitive clients
- Those who want to remain hands-on
- TinyTax for micro-entities (free)
- TaxCalc if they need more features
- Your practice's standard software if you can provide support
- Software selection advice
- Setup assistance (billable)
- Annual review of completed return (optional service)
Option 3: Training Package — Teach Them to Fish
Best for:
- Capable clients who want independence
- Groups of similar clients (efficiency)
- As a lead-in to eventual full service
- Group or 1:1 software training
- Written guides for their specific situation
- "Office hours" support package
Practical Migration Steps
Step 1: Audit Current Client Situation
For each client using CATO:
- Current year end and next filing deadline
- Complexity level (simple/moderate/complex)
- Previous years' figures (for reference)
- Likelihood of wanting professional help
Step 2: Segment and Prioritise
| Segment | Priority | Action |
|---|---|---|
| Filing due before March 2026 | High | Advise on timing — file through CATO or switch early |
| Complex affairs | High | Propose full service engagement |
| Simple, price-sensitive | Medium | Recommend free software |
| Existing partial clients | High | Propose extending engagement |
Step 3: Download History on Their Behalf
If clients authorise you:
- Access their HMRC business account
- Download all available CT600 returns
- Download computations and accounts
- Provide secure copies to client
Step 4: Set Up New Software
For clients you're bringing on:
- Set up in your practice software
- Configure HMRC connection
- Import prior year comparatives
- Guide them through TinyTax or chosen software setup
- Help with Government Gateway authorisation
- Verify first submission is successful
Software Recommendations for Different Client Types
For Clients You'll File For
Use your existing practice software (TaxCalc, IRIS, Sage, etc.)
For Independent Micro-Entity Clients
TinyTax (Recommended)
- Free for micro-entities
- Simple interface suitable for non-accountants
- Includes Companies House filing
- You could offer support package alongside
For Independent Small Company Clients
TaxCalc or similar
- More features for complex needs
- Training available
- Suitable for ongoing DIY
Pricing the Transition
One-Off Migration Services
| Service | Suggested Fee |
|---|---|
| Download HMRC history + setup in new software | £75-150 |
| Software selection consultation | £50-100 |
| Training session (1 hour) | £75-125 |
| Review of first independent submission | £50-100 |
Ongoing CT600 Service
If bringing clients on for full CT600 service:
| Client Type | Typical Range |
|---|---|
| Dormant company | £100-200/year |
| Simple trading micro | £200-400/year |
| Standard small company | £400-800/year |
| Complex / multi-entity | £800+/year |
- Time saved
- Error prevention
- Tax planning opportunities
- Peace of mind
Common Client Concerns
"Why should I pay when HMRC was free?"
Response: HMRC's service was basic and is closing. You can still use free software like TinyTax, but professional support ensures your return is correct and identifies tax savings you might miss.
"Can't I just use the free software myself?"
Response: Absolutely — TinyTax is excellent for straightforward situations. Consider professional help if your affairs are complex or you want a second pair of eyes.
"What about my old filings? Will I lose them?"
Response: Yes, access through HMRC closes. Download them now or let us do it for you. We can securely store your filing history.
"Is this a good time to start using an accountant?"
Response: It can be. The transition is a natural point to evaluate whether professional support would benefit your company. Let's discuss your specific situation.
Compliance Considerations
Agent Authorisation
Ensure proper authorisation before:
- Accessing client HMRC accounts
- Downloading filing history
- Setting up new software connections
Data Security
When handling client CT600 data:
- Secure transfer and storage
- Client access to their records
- Clear data retention policy
Professional Standards
- Don't oversell unnecessary services
- Recommend free software where appropriate
- Be transparent about what's actually changing
Timeline for Practice
| Date | Action |
|---|---|
| Now | Identify clients using CATO |
| Q1 2025 | Proactive outreach campaign |
| Q2-Q4 2025 | Migration consultations and setup |
| Jan-Mar 2026 | Final migrations before deadline |
| 31 March 2026 | HMRC service closes |
Summary
The HMRC CT600 service closure is an opportunity to:
- Bring new clients into your practice
- Strengthen relationships with existing contacts
- Demonstrate value through transition support
- Position yourself as a trusted adviser
TinyTax offers free CT600 filing for micro-entities — a good option to recommend to price-sensitive clients. Learn more →