Online Accounts and Company Tax Return Service Closing: 31 March 2026 Checklist

The online accounts and Company Tax Return service closing on 31 March 2026 is now an immediate filing risk for small UK limited companies that still use the joint HMRC and Companies House portal. From 1 April 2026, you will need commercial software to file your CT600 with HMRC, and you will no longer be able to access previous returns inside that old service. If your year-end work is usually left to the final week, this is the moment to change that habit.

Most directors are not ignoring this on purpose. They are busy running the business, and the current service has worked for years. The trouble is that this deadline is hard. When the door closes, it closes. If your software decision, data export, and filing process are still half done at the end of March, you can end up with avoidable penalties, rushed submissions, and long evenings fixing preventable mistakes.

Quick summary: pick your filing software now, save copies of previous returns before 31 March 2026, test one end-to-end filing workflow, and put all filing deadlines in one calendar owned by one named person.

If you want a quick readiness review, we can help through our annual accounts service, bookkeeping support, and contact page.

Why the online accounts and Company Tax Return service closing matters so much this month

HMRC and Companies House announced that the joint filing service closes on 31 March 2026. Up to and including that date, you can still file through it. From 1 April 2026, you cannot. You must file Company Tax Returns with HMRC using software.

That change sounds simple until you list what has to happen in real life:

• someone has to choose suitable software
• your company data has to be checked and migrated properly
• prior submissions may need to be saved for future reference
• directors and staff need to know the new filing routine
• deadlines still apply while you are changing process

Plenty of companies are already sorted. Plenty are not.

The deadline sits in a busy window too. March is year-end planning season for many owner-managed businesses, payroll teams are closing tax-year tasks, and directors are already juggling cash flow and staffing. So the filing-service closure can become the thing that gets pushed to “next week” until there is no next week left.

What changes on 1 April 2026 and what stays the same

The filing route changes. Your legal obligations do not.

Area	Before 1 April 2026	From 1 April 2026
Company Tax Return (CT600) filing	Can use HMRC online joint service (if eligible)	Must use commercial software for HMRC filing
Accounts filing to Companies House	Joint service available	Alternative filing routes remain, with software strongly expected as reforms progress
Access to previous returns in old joint service	Available until closure date	No access after closure, save copies before 31 March 2026
Filing deadlines and penalties	Normal rules apply	Same deadlines and penalties still apply

So if you were hoping deadlines might pause while you move systems, they do not.

That is why we are advising clients to treat this as a project with an owner and a short timeline, not as an admin task that can float around the to-do list.

Dates you need in your diary now

Here are the key dates most small limited companies should put in writing now.

Date	What it means	Practical action
31 March 2026	Final day the joint online filing service is available	Save prior returns and complete any final submissions in old system
1 April 2026	Joint service no longer available	Use chosen software route for HMRC Company Tax Return filing
9 months after financial year-end	Companies House accounts deadline for many private companies	Plan accounts preparation and sign-off timetable
12 months after accounting period end	CT600 filing deadline	Build software workflow to file well before deadline
9 months and 1 day after accounting period end	Corporation Tax payment deadline	Keep payment reserve and calendar reminder in place

Even if your own CT600 deadline is later in the year, the software switch is still a March job because the old service closes for everyone at once.

How to choose filing software without wasting money

There is no single “best” product for every company. What matters is fit.

Use these criteria:

• supports your company size and account type
• supports CT600 submission and Companies House needs you actually have
• gives clear audit trail and filing confirmation
• has support you can reach in busy periods
• price is sensible for your filing complexity

Start with the official GOV.UK tool for Companies House filing software, then narrow quickly.

The common error is spending weeks comparing every feature. Most small companies need a practical short list, a quick trial, and a decision. Over-analysis is expensive when a hard deadline is this close.

A quick software decision framework

Use a simple scorecard out of 5 on each item:

• filing coverage for your obligations
• ease of use for non-specialists
• support responsiveness
• migration effort from your current records
• annual total cost

Any product scoring low on filing coverage should be eliminated immediately, however cheap it looks.

Worked example 1: software decision for a micro company

Assume your company has:

• annual turnover of £180,000
• one director-shareholder
• straightforward trading activity
• no group structures or complex disclosures

You compare three options.

Option	Annual cost	Estimated setup time	Filing fit	Outcome
Option A	£180	8 hours	Full CT600 and small company accounts support	Strong candidate
Option B	£95	15 hours with manual workarounds	Partial fit only	False economy
Option C	£320	6 hours	Full fit plus accountant portal	Best if you need adviser collaboration

If Option B saves £85 but costs an extra 7 hours of director time, it is usually not a saving. If your time is worth £75 per hour, that extra time is £525.

When tax deadlines are involved, cheap software that creates manual patchwork often costs more overall.

Save your historical submissions before they disappear

One detail directors are still missing is data access. HMRC guidance says you need to save prior returns from the old service by 31 March 2026 if you want those copies for future reference.

Why does this matter?

• you may need them if HMRC asks questions later
• your future accountant may ask for earlier returns and accounts
• you may need prior figures to explain adjustments or resubmissions

Do not assume “it will still be there somewhere”. Export what you need now.

We suggest saving at least the last three years in a structured folder with clear file names:

• 2023-03-31-ct600-submission.html
• 2024-03-31-accounts-submission.html
• 2025-03-31-filing-summary.html

Also keep backup copies in cloud storage with restricted access. One copy on one laptop is not a record-keeping policy.

Penalties: what late filing can cost in pounds, not just stress

Let us turn penalties into plain numbers. These are often the wake-up call.

Company Tax Return late filing penalties (HMRC)

Current HMRC rules include:

• £100 immediately after deadline
• another £100 after 3 months
• tax-geared penalties after 6 and 12 months for unpaid tax
• higher fixed penalties if returns are late repeatedly

Companies House accounts late filing penalties (private companies)

Current Companies House penalties are:

• up to 1 month late: £150
• 1 to 3 months late: £375
• 3 to 6 months late: £750
• over 6 months late: £1,500

The penalty is doubled if accounts are late two years in a row.

These numbers are current at the time of writing and should be rechecked against GOV.UK before filing if rates are updated.

Worked example 2: one late cycle can get expensive quickly

Assume a private limited company:

• files CT600 4 months late
• files Companies House accounts 2 months late
• has unpaid Corporation Tax of £18,000 at the 6-month HMRC point

Estimated penalty exposure:

• HMRC fixed late filing penalties: £200
• HMRC 6-month tax-geared penalty: 10% of £18,000 = £1,800
• Companies House 1 to 3 months late accounts penalty: £375

Total estimated penalties: £2,375, before interest and before internal time cost.

That is money that produces no value for your business.

Worked example 3: repeat lateness doubles Companies House pain

Assume your company files accounts 20 days late in year one and 20 days late again in year two.

• Year one penalty (up to 1 month): £150
• Year two penalty doubled: £300

Two years of “just slightly late” becomes £450. The pattern is what hurts.

A practical migration plan you can run this week

If you need a plan that works in real businesses, use this.

Week 1: pick and configure software

• confirm who owns filings internally
• choose software and create secure user access
• set up company profile, accounting period dates, and filing credentials
• run a test import or trial data entry

Week 2: export old records and document the process

• download previous returns from the old service
• store files in a clear, backed-up folder structure
• write a one-page process note: who does what, when, and where evidence is stored

Week 3: run a dry-run filing workflow

• produce draft accounts from your records
• prepare CT figures in software
• check required disclosures and attachments
• run internal review and sign-off

Week 4: final checks before closure

• confirm every required historical file is saved
• confirm software login and permissions work for all relevant staff
• confirm your deadline calendar is active with reminders
• confirm who will handle questions if submission errors appear

One sentence that saves time: assign one owner. Shared ownership often becomes no ownership.

Common mistakes we are seeing right now

”We only file once a year, so we can sort this later”

Filing once a year is exactly why this catches people out. Annual tasks drift until they become urgent.

Choosing software without checking filing scope

Some tools are great for bookkeeping and weak on statutory filing. Always check your legal filing needs first.

Ignoring historic return downloads

Once the service closes, access is gone. Directors are often surprised by this.

No internal handover notes

If one staff member knows the whole process and they are on leave, filing week becomes fragile.

Leaving payments planning out of the filing plan

A return can be technically on time and still cause cash stress if the tax payment reserve is thin. Keep payment dates alongside filing dates in the same calendar.

How this links to wider Companies House reforms

The closure is part of a broader move toward stronger digital filing under UK company law reforms. Many companies will eventually be filing through software as standard.

The practical takeaway is simple. If you modernise your process now, you are not only handling the 31 March closure, you are reducing friction for future compliance changes too.

That does not mean you need an expensive finance transformation project.

It usually means:

• cleaner monthly bookkeeping
• clear month-end close checklist
• better year-end file organisation
• one reliable filing workflow with named owners

For smaller companies, that level of process is enough to cut most avoidable mistakes.

Internal links to help you implement this

If you want support before the deadline, these pages are the most relevant:

• Annual accounts service
• Bookkeeping service
• Self Assessment support for directors
• Payroll services if you need year-end coordination
• Contact Golden Tree Consulting

Authoritative sources for this guide

• Closure of the service to file your company accounts and tax return (GOV.UK)
• File your accounts and Company Tax Return (GOV.UK)
• Company Tax Returns penalties for late filing (GOV.UK)
• Prepare annual accounts for a private limited company penalties (GOV.UK)
• Find software for filing company accounts (GOV.UK)
• The online accounts and Company Tax Return service is closing (GOV.UK news)

FAQ: online accounts and Company Tax Return service closing

What exactly closes on 31 March 2026?

The joint HMRC and Companies House online service for filing company accounts and Company Tax Returns closes on 31 March 2026.

Can I still file a CT600 after 1 April 2026?

Yes, but you will need commercial software to file with HMRC after the joint service closes.

Do my filing deadlines change because the service is closing?

No. Your legal deadlines for accounts, CT600 filing, and Corporation Tax payment stay in place.

Should I download old submissions before closure?

Yes. HMRC advises saving previous returns before 31 March 2026 because you will not be able to access them in the old service after closure.

Are penalties really that significant for a small company?

They can be. Fixed penalties add up quickly, and tax-geared penalties can become expensive if returns and payments are both late.

Is this article personal tax advice for my company?

No. This is general guidance. Your filing setup and tax position depend on your exact accounts, deadlines, and company structure, so tailored advice is sensible before key filings.

Your next practical step

Block 90 minutes in your diary this week, choose your filing software, and assign one owner for the migration. If you do those three things before mid-March, the 31 March 2026 deadline becomes manageable instead of chaotic.