How to File Your CT600 Yourself in 2026 (Complete DIY Guide)

You don't need an accountant to file your CT600. Thousands of company directors do it themselves every year — and with HMRC's free filing tool closing in March 2026, the process is actually getting easier thanks to modern filing software.

Here's exactly how to do it.

Who Can File Their Own CT600?

If your company matches most of these criteria, you can almost certainly file yourself:

✅ Small limited company (turnover under £632,000)
✅ One or two directors (typically the shareholders too)
✅ Simple income — trading income, maybe some bank interest
✅ Standard expenses — salaries, rent, supplies, professional fees
✅ No complex structures — no group companies, overseas income, or R&D claims
✅ FRS 105 micro-entity accounts (balance sheet total under £316,000)

Reality check: The vast majority of UK limited companies qualify as micro-entities. If you're a typical contractor, freelancer, or small business owner, filing yourself is absolutely doable.

What You'll Need Before You Start

Gather these before you begin:

Financial records

Bank statements for the full accounting period
Sales invoices — every invoice you issued
Purchase receipts — every business expense
Payroll records — salary payments, PAYE, NI
Dividend records — board minutes and dividend vouchers
Asset purchases — equipment, computers, vehicles

Don't have perfect records?

Bank statements are your safety net. Every business transaction flows through your bank account. Work backwards from there if your invoice/receipt records are incomplete.

Step 1: Calculate Your Profit

Total income

Add up all money your company earned during the accounting period:

Invoice payments received
Bank interest
Any other income

Total expenses

Add up all allowable business expenses:

Staff costs (salaries, employer NI, pension contributions)
Rent and utilities
Professional fees (accountant, legal, insurance)
Travel and subsistence (business trips only)
Office supplies, software subscriptions
Marketing and advertising
Bank charges
Phone and internet (business proportion)

Adjustments

Some accounting items need adjusting for tax:

Add back depreciation (not tax-deductible — use capital allowances instead)
Add back entertaining (client entertainment isn't deductible)
Add back fines and penalties
Deduct capital allowances (AIA, full expensing, or WDA)

Your taxable profit

Income - Expenses + Adjustments = Taxable Profit

Step 2: Calculate Corporation Tax

Simple calculation (all profit in one financial year)

If your accounting period falls entirely within one financial year (e.g., 1 April 2025 to 31 March 2026):

Profit Level	Rate
Up to £50,000	19%
£50,001 to £250,000	Marginal relief rate (~26.5% effective)
Over £250,000	25%

Example: Profit of £40,000

Tax: £40,000 × 19% = £7,600

Example: Profit of £100,000

First £50,000 at 19%: £9,500
Marginal relief on next £50,000: ~£13,250
Total: ~£22,750

Straddling financial years

If your period straddles two financial years (e.g., calendar year 1 January to 31 December), you apportion profits between the two years and apply each year's rates.

Taxpipe handles this automatically. Our calculator does the financial year split, marginal relief, and associated company adjustments for you.

Step 3: Prepare Your Accounts

Your CT600 must be accompanied by statutory accounts in iXBRL format. For most small companies, this means micro-entity accounts under FRS 105.

What micro-entity accounts include

Balance sheet — assets, liabilities, equity at year-end
Profit & loss statement (income statement)
Notes (minimal for micro-entities)

The iXBRL requirement

HMRC requires accounts in inline XBRL format — machine-readable XML embedded in an HTML document. You can't just upload a PDF.

This is where filing software is essential. Software like Taxpipe generates your iXBRL accounts automatically from the figures you enter.

Balance sheet items

Item	What to include
Cash at bank	Balance on your bank statement at year-end
Debtors	Money owed to you by customers
Fixed assets	Equipment, vehicles (at written-down value)
Creditors	Money you owe (bills, credit cards, tax due)
Director's loan	Balance of director's loan account
Share capital	Usually £1 (1 ordinary share)
Retained profit	Cumulative profit minus dividends minus tax

Step 4: Fill In the CT600

The CT600 has many boxes, but most small companies only use a few. For a complete walkthrough of every box, see our CT600 box-by-box guide.

Key boxes for a simple return

Box	What It Is	Example
1	Total turnover	£120,000
3	Trading profits	£45,000
21	Total profits chargeable	£45,000
37	Corporation tax	£8,550
38	Marginal relief (if applicable)	—
40	Net CT payable	£8,550
145	Tax payable	£8,550
155	Tax outstanding	£8,550
430	Turnover from trade	£120,000
435	Trading profits	£45,000
660	Capital allowances	£3,000

Boxes you can usually leave blank

Boxes related to group relief (unless you're in a group)
Boxes related to overseas income (unless you have foreign earnings)
Boxes related to R&D (unless you're claiming research relief)
Boxes related to property (unless the company has rental income)

Step 5: Submit to HMRC

Using filing software (recommended)

Filing software like Taxpipe handles the submission:

Enter your figures in the guided wizard
Review the generated CT600 and accounts
Click "Submit to HMRC"
Get your confirmation reference

What happens after submission

Immediate acknowledgement — HMRC confirms receipt
1-3 working days — HMRC validates and accepts (or rejects with error codes)
You're done — keep the confirmation reference for your records

Step 6: Pay Your Corporation Tax

Payment is due 9 months and 1 day after your accounting period ends. Pay via:

Online banking (Faster Payments) — use your 17-character payment reference
BACS — 3 working days to process
Direct Debit — set up through HMRC

Your payment reference is your UTR followed by the accounting period end date (AYYMMDD format).

Common DIY Mistakes to Avoid

1. Claiming personal expenses

Only claim expenses that are wholly and exclusively for business. No groceries, personal phone bills, or family holidays — even if you "did some work."

2. Forgetting to add back depreciation

Your accounting software might include depreciation in your expenses. For tax purposes, you must add this back and claim capital allowances instead.

3. Not claiming capital allowances

If you bought equipment, computers, or vehicles, claim the Annual Investment Allowance (100% deduction up to £1 million). Many DIY filers miss this.

4. Wrong accounting period

Make sure your CT600 dates match what HMRC expects. If your first period is longer than 12 months, you need two CT600s.

5. Missing the payment deadline

You must pay within 9 months + 1 day, but file within 12 months. Payment comes first — don't confuse them. See our CT600 deadline guide for the exact dates.

6. Not keeping records

HMRC can enquire into your return for up to 12 months after filing (longer for errors). Keep all receipts, bank statements, and working papers for 6 years.

How Long Does It Take?

First time filing

Gathering records: 1-2 hours
Preparing accounts: 1-2 hours
Filling in the CT600: 30-60 minutes
Submission: 5 minutes
Total: 3-5 hours for your first return

Subsequent years

Once you know the process and have a system for records:

Total: 1-2 hours

With Taxpipe's guided wizard, the CT600 itself takes about 15 minutes — the preparation time is mostly gathering your records.

When Should You NOT DIY?

Consider using an accountant if:

Your company has overseas income or subsidiaries
You're claiming R&D tax relief (specialist claims)
You have complex share structures or multiple shareholders
The company is part of a group claiming group relief
You're being investigated by HMRC
You're not confident in your figures and want professional review

The Cost Comparison

Method	Cost	Time
Accountant	£300-£1,000+	Weeks (their turnaround)
DIY with Taxpipe	£59	1-2 hours (your time)
DIY with HMRC free tool	Free (until March 2026)	2-4 hours (clunkier interface)

Frequently Asked Questions

Is it legal to file my own CT600?

Absolutely. There's no requirement to use an accountant. Any company officer (director) can file the CT600.

What if I make a mistake?

You can amend your CT600 within 12 months of the filing deadline. If you overpaid tax, HMRC will refund the difference.

Will HMRC judge me for filing without an accountant?

No. HMRC processes all returns the same way regardless of who prepared them. They're looking at the figures, not who filed.

Can I claim my filing costs as a business expense?

Yes. The £59 Taxpipe fee (or any accountancy fee) is a deductible business expense on your next CT600.

Ready to file your CT600 yourself? Start with Taxpipe — guided entry, automatic calculations, direct HMRC submission. Just £59, no accountant needed.