Based on verified HMRC and IRS regulations for January 2026
Filing a business tax return is one of the most important financial tasks of the year—and also one of the most confusing. Rules differ between the UK and US, deadlines vary by business type, and new 2026 changes mean many small businesses must switch to digital reporting for the first time.
This guide simplifies everything into one clear resource.
You’ll learn:
- Who needs to file a business tax return
- UK & US deadlines for 2025/26
- The forms you must submit
- Allowable vs not-allowable expenses
- How to file step-by-step
- 2026 digital filing changes (VERY IMPORTANT)
- Common mistakes business owners make
- Tools & software to use this year
Whether you’re a sole trader, LLC owner, freelancer, or limited company director—this guide covers you.
Quick Takeaways (2026 Snapshot)
- UK Self Assessment Deadline: 31 January 2026
- US Sole Proprietor Deadline: 15 April 2026
- US Partnership Deadline: 15 March 2026
- UK Corporation Tax Filing: 12 months after year-end
- UK Corporation Tax Payment: 9 months + 1 day
- New 2026 Rule: HMRC is discontinuing free CATO CT600 filing from April 2026
- New 2026 Rule: MTD for Income Tax becomes mandatory from 6 April 2026 for earners over £50k
What Is a Business Tax Return? (Simple Definition)
A business tax return is an official document that reports your:
- income
- business expenses
- profit or loss
- tax owed
In the UK, returns go to HMRC.
In the US, returns go to the IRS.
Even if your business made little or no profit, you still may need to file.
Who Needs to File? (UK vs US)
Here’s the clean, corrected version based on 2026 rules:
🇬🇧 UK — Who Must File
You must file if you are:
✔ Sole trader earning over £1,000
✔ Partner in a business partnership
✔ A limited company (must file CT600 even at £0 profit)
✔ Director earning income outside PAYE
✔ Landlord earning over £1,000
⚠ Clarification:
The company files CT600, not the director personally—unless the director also needs Self Assessment.
🇺🇸 US — Who Must File
You must file if you are:
✔ Sole proprietor filing Schedule C
✔ Partnership filing Form 1065
✔ C-Corporation filing Form 1120
✔ S-Corporation filing Form 1120-S
✔ LLC with tax elections (varies by structure)
US rules depend on how the business is taxed, not how it is formed.
UK & US Business Tax Deadlines (2026 Updated)
🇬🇧 UK Deadlines
| Filing Type | Deadline |
|---|---|
| Self Assessment | 31 January 2026 |
| Corporation Tax Payment | 9 months + 1 day after year-end |
| CT600 Filing | 12 months after the accounting period ends |
| VAT Return | Usually quarterly |
| PAYE | Monthly or quarterly |
⚠ 2026 Alert: MTD Begins April 2026
If you earn over £50,000, Making Tax Digital for Income Tax becomes mandatory from 6 April 2026.
🇺🇸 US Deadlines
| Entity Type | Deadline 2026 | Notes |
|---|---|---|
| Sole Proprietor (Schedule C) | April 15, 2026 | Part of Form 1040 |
| Partnership (1065) | March 15, 2026 | K-1s issued to partners |
| S-Corporation (1120-S) | March 15, 2026 | |
| C-Corporation (1120) | April 15 OR 15th day of the 4th month after year-end | Corrected wording |
| LLC | Varies | Based on tax election |
Required Forms (Side-by-Side Guide)
🇬🇧 UK Forms
- SA100 — Sole trader tax return
- SA800 — Partnership return
- CT600 — Corporation Tax return
- Annual accounts — For limited companies
🇺🇸 US Forms
- Schedule C — Sole proprietors
- 1065 — Partnerships
- 1120 — C-Corps
- 1120-S — S-Corps
- K-1 — Partners/shareholders
Allowable vs Not Allowable Expenses
🇬🇧 UK Allowable Expenses
✔ Marketing and advertising
✔ Software & subscriptions
✔ Office costs
✔ Staff salaries
✔ Professional fees
✔ Business travel
❌ Not Allowable
✘ Client entertainment
✘ Personal spending
✘ Clothing (unless protective gear)
🇺🇸 US Allowable Expenses
✔ Home office deduction
✔ Depreciation
✔ Contractor payments
✔ Insurance
✔ Supplies
❌ Not Allowable
✘ Personal meals
✘ Commuting
✘ Clothing used outside of business
Also Check: How to File an HMRC Tax Return Online (Step by Step)
Example: How Business Tax is Calculated
🇬🇧 UK Corporation Tax Example
If a company earns £50,000 profit:
£50,000 × 25% = £12,500 tax
(Assuming main corporation tax rate applies.)
🇺🇸 US Example
- Sole proprietors pay through personal income tax
- C-Corps pay 21% federal corporate tax
- S-Corps pass income to owners (no corporate tax)
⚠️ 2026 Mandatory Digital Filing Updates (Important)
🇬🇧 UK — Two Major Changes
1. Free HMRC CATO filing ends April 2026
HMRC will discontinue its free CT600 filing tool.
👉 All companies must use commercial software like Sage, Xero, FreeAgent, or a CT600-specific tool.
2. MTD for Income Tax starts April 2026
Affects taxpayers earning over £50,000.
You must:
- keep digital records
- Use approved MTD software
- Submit quarterly updates
This is the last year of the old annual system for high earners.
The 3-Part Business Tax Return System™ (For Simpler Filing)
1️⃣ Record Everything
Keep receipts, invoices, bank statements, and digital logs.
2️⃣ Calculate Profit
Profit = Revenue – Allowable Expenses.
3️⃣ File & Pay
Submit the required forms by the deadline and make payment.
How to File a Business Tax Return (Step-by-Step)
Step 1 — Gather Documents
- bank statements
- invoices
- receipts
- payroll data
- software reports
Step 2 — Calculate Income & Expenses
Use accounting software to avoid errors.
Step 3 — Prepare Required Forms
Based on entity type (UK/US tables above).
Step 4 — File Online
Using HMRC/IRS or approved software.
Step 5 — Pay Tax Owed
Before deadlines to avoid penalties.
Common Mistakes Business Owners Make (and How to Avoid Them)
❌ Missing the payment deadline
❌ Mixing personal and business expenses
❌ Forgetting to depreciate assets
❌ Using the wrong business structure
❌ Not registering for Self Assessment in time
❌ Filing CT600 late, triggering automatic penalties
❌ Forgetting US estimated quarterly tax payments
Also Read: Beat the Tax Year Chaos: UK Dates 2025/26 & 2026/27 Every Brit Needs
2026 Business Tax Filing Software Comparison (UK & US — Verified Capabilities)
| Software | Country | Best For | Supported Tax Filings | Digital Compliance |
|---|---|---|---|---|
| Xero | UK & US | Small & mid‑sized businesses | UK: Self Assessment, CT600 (via partners) / US: Schedule C, 1120, 1120‑S (via export) | UK: MTD compatible with add‑ons |
| Sage Business Cloud | UK | SMEs & established businesses | CT600, Self Assessment, PAYE, VAT | MTD compatible |
| FreeAgent | UK | Freelancers & contractors | Self Assessment, expense tracking integration | MTD compatible |
| TaxCalc | UK | Accountants & complex returns | CT600, Self Assessment | MTD compatible |
| GoSimpleCT600 | UK | Limited companies needing CT600 only | CT600 filing | Limited — MTD via add‑ins |
| QuickBooks Online | US & UK | Small business bookkeeping + tax prep | US: Schedule C, 1065, 1120, 1120‑S / UK export for tax reporting | UK: Works with MTD plugins |
| TurboTax Business | US | Corporations, LLCs & partnerships | US: 1065, 1120, 1120‑S, Schedule C | N/A (IRS e‑file compliant) |
| H&R Block | US | DIY business tax filers | Schedule C, 1065, 1120, 1120‑S | N/A (IRS e‑file compliant) |
| Drake Tax | US | Professional preparers | Full suite: Schedule C, 1065, 1120, 1120‑S | N/A (IRS compliant) |
| TaxAct | US | Value‑oriented tax filing | Schedule C, 1065, 1120, 1120‑S | N/A (IRS compliant) |
FAQs
Q1. When should a business submit a tax return in the UK and US?
A business must submit its tax return by the official deadline:
- UK: 31 January 2026 for Self Assessment (2024/25 tax year)
- US: March 15 for partnerships and S-Corps, April 15 for sole proprietors and most C-Corps (or the 15th day of the 4th month after the fiscal year-end)
Submitting on time helps avoid penalties and interest.
Q2. What happens if I file my business tax return late?
Filing a business tax return late in the UK or US can result in:
- Automatic fines and late fees
- Interest on unpaid taxes
- Possible increased scrutiny from HMRC (UK) or IRS (US)
Even if your business made no profit, filing late can still trigger penalties.
Q3. Do I need an accountant to file a business tax return?
No, hiring an accountant is not legally required, but it’s highly recommended for:
- Limited companies, S-Corps, or C-Corps
- Cross-border business owners or digital nomads
- Businesses with complex expenses or multiple revenue streams
Using an accountant or professional software reduces errors and ensures compliance with UK HMRC and US IRS requirements.
Q4. Can I file my business taxes if my business made no profit?
Yes. Zero-profit businesses still need to submit a business tax return:
- UK: Limited companies must file CT600 even if profit is £0
- US: Sole proprietors, partnerships, and corporations must still report income and expenses
Filing ensures your business remains compliant and avoids unnecessary penalties.
Q5. How do I know which forms to file for my business in 2026?
The forms depend on your business structure:
- UK Sole Trader: SA100 (Self Assessment)
- UK Limited Company: CT600 + annual accounts
- US Sole Proprietor: Schedule C (Form 1040)
- US Partnership: Form 1065
- US C-Corp: Form 1120
- US S-Corp: Form 1120-S
Use official HMRC and IRS guidance or trusted tax software to ensure the correct forms are filed.
Q6. Can I submit my business tax return online?
Yes, most businesses can file online:
- UK: HMRC portal or MTD-compatible software (mandatory for certain earners from April 2026)
- US: IRS e-file system or commercial tax software like TurboTax, QuickBooks, H&R Block
Online filing is faster, reduces errors, and provides immediate confirmation of submission.
Related: Deadline for Income Tax Return 2025–2026: UK Filing Guide
