Corporate Tax Filing: Common Mistakes to Avoid

Tax filing mistakes can be costly. Here are the most common errors we see—and how to avoid them.

 

Mistake #1: Missing the Deadline

The Error: Filing after the 150-day deadline (or May 31 for e-filing).

Consequences:

• Late filing penalty: 1,000-2,000 THB
• Late payment penalty: 1.5% per month on unpaid tax
• Interest: Compounds monthly

Example:

• Tax owed: 500,000 THB
• 3 months late
• Penalty: 500,000 × 1.5% × 3 = 22,500 THB
• Plus: Late filing penalty

 

How to Avoid:

• Mark deadline on calendar
• Start preparation 3 months early
• Use e-filing for extended deadline (May 31)
• Set reminders at 60, 30, and 7 days before

 

Mistake #2: Incorrect Revenue Recognition

The Error:

• Recording revenue in wrong year
• Missing accrued revenue
• Double-counting revenue

Example:

• Invoice dated Dec 28, 2025
• Payment received Jan 5, 2026
• Correct: Record in 2025 (accrual basis)
• Wrong: Record in 2026 (when paid)

 

How to Avoid:

• Use accrual accounting (not cash basis)
• Review year-end cut-off carefully
• Reconcile all invoices
• Work with experienced accountant

 

Mistake #3: Non-Deductible Expenses Claimed

The Error: Claiming expenses that don't qualify for deduction.

Common non-deductible items:

• Personal expenses (family meals, personal car)
• Entertainment without proper documentation
• Fines and penalties
• Donations to non-approved charities
• Capital expenditures (must depreciate)

 

How to Avoid:

• Separate business and personal expenses
• Keep detailed receipts with business purpose
• Know what's deductible vs. depreciable
• Review expense categories before filing

 

Mistake #4: Missing Withholding Tax Credits

The Error: Not claiming credit for withholding tax already paid.

Example:

• Your company earned 100K THB interest
• Bank withheld 15K THB (15%)
• You file tax return but forget to claim the 15K credit
• Result: You pay tax twice on the same income

 

How to Avoid:

• Collect all withholding tax certificates
• File them with tax return
• Reconcile credits against tax liability
• Keep certificates for 5 years

 

Mistake #5: Improper BOI Activity Separation

The Error: BOI companies not separating promoted vs. non-promoted revenue.

Example:

• Software development (promoted): 8M THB
• Consulting (non-promoted): 2M THB
• Wrong: Report all 10M as promoted (0% tax)
• Correct: Separate accounting, tax non-promoted portion

 

How to Avoid:

• Maintain separate books for each activity
• Use different invoice series
• Allocate shared costs properly
• Document allocation methodology

 

Mistake #6: Forgetting 200% Deductions

The Error: Not claiming enhanced deductions for training and R&D.

Lost opportunity:

• Spent 200K on training
• Claimed: 200K deduction (standard)
• Should have claimed: 400K deduction (200%)
• Lost tax savings: 40K THB

 

How to Avoid:

• Track training and R&D expenses separately
• Obtain approval letters in advance
• Maintain proper documentation
• Review deduction opportunities before filing

 

Mistake #7: Incorrect SME Status

The Error: Claiming SME rates when you don't qualify.

Disqualification triggers:

• Capital exceeds 5M THB
• Revenue exceeds 30M THB
• Became BOI-promoted

 

How to Avoid:

• Verify capital and revenue annually
• Update status when thresholds crossed
• Don't claim SME rates if ineligible
• Consult accountant if near thresholds

 

Mistake #8: Poor Documentation

The Error: Missing receipts, invoices, or supporting documents.

Audit scenario:

• Revenue Department requests proof of expenses
• You can't provide receipts for 500K THB
• Result: 500K disallowed, tax recalculated
• Additional tax: 100K THB + penalties

 

How to Avoid:

• Scan all receipts immediately
• Organize by month and category
• Use cloud storage (backup)
• Keep for 5 years minimum

 

Mistake #9: Not Reconciling with VAT Returns

The Error: Revenue on CIT return doesn't match VAT returns.

Red flag:

• VAT returns show 10M THB revenue
• CIT return shows 8M THB revenue
• Triggers: Automatic audit

 

How to Avoid:

• Reconcile CIT and VAT before filing
• Explain any differences (exempt sales, etc.)
• Ensure consistency across all filings

 

Mistake #10: DIY Without Expertise

The Error: Filing complex returns without professional help.

Risk areas:

• International transactions
• BOI accounting
• Multiple revenue streams
• Large deductions

 

How to Avoid:

• Hire qualified accountant
• Cost: 15,000-30,000 THB/year
• Savings from avoiding mistakes: Often 100,000+ THB

 

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Related Service: Accounting & Tax Compliance — Avoid all these mistakes with professional tax filing.