Compiled by the YMTD Accounting Partner team · Information as of 6 October 2026

If you are a business owner asking yourself "I haven't done my accounting for months, what should I do?" as the third quarter (Q3) of 2026 arrives, the urgent fix is to gather the bank statements of every account and prepare a bank reconciliation to separate income from expenses. Then file the overdue withholding tax returns (P.N.D.1, 3, 53) and VAT returns (P.P.30) promptly to stop the surcharge of 1.5% per month [2], and file an additional P.N.D.51 half-year return before the end of the accounting period to avoid the 20% surcharge under Section 67 ter of the Revenue Code [3].

Key takeaways

  • Stop the 1.5% monthly surcharge immediately: Every type of unpaid tax carries a surcharge of 1.5% per month (a fraction of a month counts as one month) under Section 27 of the Revenue Code [2]. Filing the overdue returns yourself as soon as possible keeps the surcharge from snowballing.
  • File an additional P.N.D.51 before year-end to avoid the 20% surcharge: If you file an additional P.N.D.51 and pay the tax before the deadline for the annual P.N.D.50 return, you pay only the 1.5% monthly surcharge and are not liable for the 20% surcharge on the tax underpaid under Section 67 ter [3].
  • Expenses without a tax invoice can still be deducted: Genuine business expenses that have no tax invoice can be recorded as tax-deductible expenses using a payment voucher with a copy of the payee's ID card, a transfer slip, or a certificate in lieu of a receipt, without having to add them back as non-deductible expenses under Section 65 ter (13).
  • Fines for financial statements not filed with the DBD: Failing to keep accounts and failing to file annual financial statements is punishable by a pinai (civil) fine under the Accounting Act of 2000 together with the Act on Pinai Fines of 2022, of up to 50,000 baht, for both the juristic person and the authorised director [5].

Introduction and scope of the problem: the impact of not keeping accounts through to Q3

For small and medium-sized enterprises (SMEs), the first half of the year is usually focused on sales and running the storefront, so the monthly accounting and tax paperwork can be neglected. By the third quarter (July to September), a business has often accumulated a large pile of receipts, tax invoices and bank transactions that have not been recorded.

Letting the accounting backlog run into Q3 directly affects the financial position and tax compliance of the business in three main ways:

  • Risk of missing the half-year corporate income tax return (P.N.D.51): The P.N.D.51 must be filed within 2 months after the last day of the first 6-month period (31 August for a calendar-year period). Without real accounting figures, the business cannot prepare an estimate of net profit, which leads to a missed filing deadline or an estimate that falls short of the actual net profit by more than 25% [3].
  • Unpaid taxes and accumulating monthly surcharges: VAT (P.P.30) and withholding tax (P.N.D.1, P.N.D.3, P.N.D.53) must be remitted every month. If they remain unpaid for 7–9 months, every period carries both a criminal fine for late filing and a surcharge of 1.5% per month [1, 2].
  • Risk of losing the right to claim input tax: Under the Revenue Code, input tax shown on a full tax invoice can be offset against output tax within 6 months from the month the tax invoice was issued. If this is neglected beyond 6 months, the business loses the right to credit that input tax in the VAT system [4].

Review of the relevant laws and tax rules

Catch-up bookkeeping and amended tax filings must follow the framework of the Revenue Code and the accounting profession laws. The key rules are as follows:

1. Criminal fines and surcharges for late filing

Failing to file a tax return within the statutory deadline (P.N.D.50, P.N.D.51, P.P.30, P.N.D.1, P.N.D.3, P.N.D.53) carries a criminal fine of up to 2,000 baht under Section 35 of the Revenue Code [1]. The Revenue Department's settlement fine is generally set at 1,000 to 2,000 baht, depending on how late the return is filed.

As for the surcharge, if there is tax payable, the business must pay a surcharge of 1.5% per month or fraction of a month on the tax due, with a fraction of a month counted as a full month under Section 27 of the Revenue Code. The accumulated surcharge is capped at the amount of tax payable [2].

2. Surcharge rules for an underestimated net profit under Section 67 ter

Under Section 67 bis of the Revenue Code, a company or juristic partnership must prepare an estimate of its net profit or net loss for the whole year and file a P.N.D.51 return, paying half of the estimated tax, within 2 months after the end of the first 6-month period. If the business does not file, or files an estimate of net profit that falls short of the actual net profit for the year by more than 25% without reasonable cause, it must pay a surcharge of 20% of the tax underpaid under Section 67 ter [3].

However, under Revenue Department Instruction No. Tor.Por. 81/2542, if the operator finds that it did not file or estimated too low, and files an additional P.N.D.51 with full payment of the tax before filing the annual P.N.D.50, the business pays only the 1.5% monthly surcharge on the additional tax under Section 27 and is not liable for the 20% surcharge under Section 67 ter [2, 3]. See further details in our guide on how to calculate P.N.D.51 fines and reduce the surcharge.

3. VAT penalties and surcharges

If a VAT-registered operator has not filed P.P.30 returns, the law sets a penalty under Section 89 of the Revenue Code of up to 2 times the tax payable [4]. However, if the operator files the returns and pays the additional tax voluntarily before a revenue officer issues a summons for an audit, the Revenue Department has regulations under Section 89/1 that reduce the penalty in proportion to how late the filing is [4].

4. Duties under the Accounting Act of 2000

A juristic person must appoint a bookkeeper with the qualifications required by law, and must prepare financial statements and file them with the Department of Business Development (DBD) within 5 months from the closing date of the accounting period under Sections 11 and 19 of the Accounting Act of 2000. Failing to keep accounts and failing to file financial statements is punishable by a pinai (civil) fine under the Act on Pinai Fines of 2022 of up to 50,000 baht, for both the juristic person and the authorised signing director [5].

5 steps to clear the Q3 backlog and what to do when accounts have not been kept for months

Clearing an accounting backlog (backlog accounting) is the process of gathering, checking, confirming balances and recording financial transactions retroactively from the start of the accounting period to the current period, so that the financial position is corrected and overdue taxes are filed correctly according to the law.

The 5-step approach for clearing the Q3 backlog is as follows:

Step 1: Gather the statements of every account and prepare a 9-month bank reconciliation

Download the electronic bank statements of every deposit account in the company's name from 1 January 2026 to the end of Q3, and prepare a bank reconciliation against transfer slips and evidence of receipts and payments, sorting the transactions into 3 main types:

  • Operating income: Match incoming funds to invoices, receipts and the output tax report to catch any income that was missed.
  • Purchases of goods and operating expenses: Check whether each outgoing transaction has a tax invoice, a receipt or complete evidence of payment.
  • Items that are not operating income or expenses: For example, capital contributions from shareholders, or loans between a director and the company, which require a loan agreement and interest charged according to Revenue Department criteria.

Step 2: Sort and categorise the monthly tax documents (VAT and WHT)

Separate the documents by month from January to September, dividing the taxes into 2 groups:

  • Withholding tax (P.N.D.1, P.N.D.3, P.N.D.53): Gather the payments of wages, salaries, service fees, contract work, advertising and rent, prepare the withholding tax certificates (under Section 50 bis), and summarise the tax to be remitted for each month.
  • VAT (P.P.30): Check the sales tax invoices and purchase tax invoices (input tax). A full-form purchase tax invoice under Section 86/4 can be deducted from output tax on the P.P.30 return within 6 months from the month the document was issued [4].

Step 3: Resolve expenses with no receipt or incomplete documents

Where money has been paid out of the account but there is no tax invoice or receipt, handle it under the Revenue Code rules to avoid it being assessed as a non-deductible expense under Section 65 ter, as follows:

  • Where the payee is an individual (e.g. contract work, service fees): Prepare a payment voucher, attach a copy of the payee's ID card signed to acknowledge receipt of the money, attach the bank transfer slip as evidence, and withhold tax at the rate required by law.
  • Where the expense is a small miscellaneous item: Have the employee who made the payment prepare a certificate in lieu of a receipt (Form Bor.Chor.1) certifying that the payment was actually made for the benefit of the business.
  • Where the payee's identity cannot be proven: The item must be recorded as an accounting expense but added back as a non-deductible expense when computing net profit for annual corporate income tax, following the guidelines on handling non-deductible expenses for tax.

Step 4: Calculate the tax and file the overdue returns to stop the surcharge

Once the reconciliation is done and the documents are fully sorted, file the overdue tax returns through the Revenue Department's e-Filing system in order of priority:

  1. File the overdue P.N.D.1, P.N.D.3 and P.N.D.53 returns for the periods in which income was paid, paying the tax plus the 1.5% monthly surcharge [2]
  2. File the overdue monthly P.P.30 returns, pay the tax difference plus the 1.5% monthly surcharge, and calculate the penalty for which a reduction is requested under Section 89/1 [4]
  3. File an additional P.N.D.51 and pay the half-year tax to stop the surcharge and discharge the liability under Section 67 ter [3]

A detailed list of the documents you need to prepare is in our checklist for catch-up bookkeeping.

Step 5: Plan Q4 taxes and set up a new bookkeeping system

Once the accounting entries for the first 9 months have been corrected, the business will know its true accounting and taxable net profit as at the end of Q3. It can then estimate the tax burden for the whole year and plan Q4 taxes correctly, for example by managing inventory, investing in assets for depreciation under Section 65 bis (2), and setting up a digital document system so that documents are handed to the accountant every month on a regular basis.

Analysis and comparison: options for handling backlogged documents and expenses without a tax invoice

Handling incomplete accounting documents correctly reduces the risk of a retroactive tax assessment. The table below compares the correct approach with the risk of neglecting the documents:

Type of item Correct handling under the law Tax effect (if handled correctly) Risk (if neglected or not handled)
Service fees paid to an individual (no receipt) Prepare a payment voucher + withhold tax (P.N.D.3) + attach a copy of the ID card and the transfer evidence Fully deductible as a tax expense under Section 65 bis Treated as a non-deductible expense under Section 65 ter (13); must be added back, increasing corporate income tax
Unclaimed input tax not older than 6 months Record it in the input tax report and claim the credit on the current P.P.30 return under Section 82/3 [4] Input tax is offset against output tax as entitled, reducing the VAT actually payable After 6 months the right to credit the input tax is lost; it can only be recorded as an accounting expense
P.N.D.51 not filed in August Calculate the 9-month profit, prepare the full-year estimate, and file an additional P.N.D.51 in Q3 Surcharge of 1.5% per month only on the additional amount paid [2] If left until the P.N.D.50 is filed, a 20% surcharge on the tax underpaid applies under Section 67 ter [3]
Incoming transfers of unknown origin Review the source; if it is a director's loan, prepare a loan agreement and a supporting board resolution Loans are separated from sales income and are not included in VAT or income tax computations The Revenue Department may assess it as missed sales income, with 7% VAT, a penalty of up to 2 times, and surcharges [4]

Case study: a hypothetical example of clearing a 9-month backlog for a business in Chiang Mai

(Note: the following example is a model to illustrate the steps for calculating tax, criminal fines and surcharges under the Revenue Code)

Chiang Mai Craft Products Co., Ltd. sells handicraft products in Mueang District, Chiang Mai. It is VAT-registered and its accounting period runs from 1 January to 31 December 2026. The company did not keep accounts and did not file any tax returns from January to September 2026 (9 months in total), and began the catch-up bookkeeping process on 6 October 2026.

Financial data gathered from the bank reconciliation (9-month period):

  • Total sales: 4,280,000 baht (including 7% VAT, i.e. sales before VAT of 4,000,000 baht and output tax of 280,000 baht)
  • Input tax with legally valid tax invoices: 140,000 baht
  • Contract work paid to individuals (supported by payment vouchers, 3% withholding tax not yet deducted): 300,000 baht (withholding tax outstanding of 9,000 baht)
  • Estimated net profit for the whole of 2026 after adjustments: 800,000 baht (SME corporate tax rates: the first 300,000 baht of profit is exempt; profit of 300,001 - 800,000 baht is taxed at 15%, giving income tax for the year of 75,000 baht)

Steps for calculating the tax and surcharges payable retroactively:

1. Withholding tax (P.N.D.3, 9 months overdue):

  • Tax outstanding: 9,000 baht
  • Criminal fine for late filing: an average of 200 baht per month for 9 periods = 1,800 baht [1]
  • Surcharge of 1.5% per month (based on how long each period has been overdue, 4 months on average): 9,000 × 1.5% × 4 = 540 baht [2]
  • Total tax and fines for P.N.D.3 = 11,340 baht

2. VAT (P.P.30, 9 months overdue):

  • VAT payable (output tax 280,000 - input tax 140,000) = 140,000 baht
  • Criminal fine for late filing: 9 periods × 500 baht = 4,500 baht [1]
  • Surcharge of 1.5% per month (4.5 months on average): 140,000 × 1.5% × 4.5 = 9,450 baht [2]
  • Penalty: filing and paying voluntarily before the officer issues a summons, with a reduction under Section 89/1 to 20% of the full penalty = 28,000 baht [4]
  • Total tax and fines for P.P.30 = 181,950 baht

3. Half-year corporate income tax (additional P.N.D.51 filed in October 2026):

  • Half of the estimated tax for the year: 75,000 ÷ 2 = 37,500 baht
  • Criminal fine for late filing (filed after the 31 August 2026 deadline): 2,000 baht [1]
  • Surcharge of 1.5% per month (2 months late, September and October): 37,500 × 1.5% × 2 = 1,125 baht [2]
  • Benefit obtained: Filing the P.N.D.51 in October means the company does not have to pay the 20% surcharge under Section 67 ter, which, if left until the annual P.N.D.50 is filed, would have been as much as 7,500 baht (37,500 × 20%) [3]
  • Total tax and fines for P.N.D.51 = 40,625 baht

Summary of the calculation: Clearing the backlog voluntarily in Q3 lets the company avoid the full penalty and eliminate the 20% surcharge under Section 67 ter, a saving of more than 100,000 baht compared with waiting for the Revenue Department to issue an audit summons [3, 4].

How to choose an accounting services specialist in Chiang Mai and check a bookkeeper's status

If the business owner is short of time and tax expertise, hiring an accounting firm to clear the backlog is an efficient option. To be confident in the quality of the work and the standard of accountability, check the following qualifications of the bookkeeper:

  1. Check the bookkeeper's status on the DBD e-Accountant system: Verify that the accountant is correctly registered under the Accounting Act of 2000 and has active continuing professional development (CPD) status under the criteria of the Federation of Accounting Professions (TFAC) [5]
  2. Choose a provider that uses digital accounting and document systems: The accounting firm should have a system that links bank statements and transfer slips through the cloud, such as FlowAccount or standard accounting software, to reduce errors and prevent documents from being lost.
  3. Check the scope of the backlog-clearing service: The engagement contract should cover bank reconciliation, preparation of the fixed asset register, filing of amended returns for every type of tax, and preparation of the financial statements for the certified public accountant (CPA) to audit and sign.

Business owners in the northern region can consider using a specialist that provides accounting services in Chiang Mai, so that original supporting documents can be checked at the business premises quickly and on an ongoing basis.

Frequently asked questions (FAQ)

I set up a company and left it idle, never kept accounts and never filed taxes. How much will the fines be?

Leaving a company idle without keeping accounts or filing tax returns carries a criminal fine from the Revenue Department of up to 2,000 baht per return under Section 35 [1], plus a surcharge of 1.5% per month on the unpaid tax under Section 27 [2], and a pinai (civil) fine from the Department of Business Development (DBD) for failing to file financial statements of up to 50,000 baht, for both the juristic person and the authorised director under the law [5].

I forgot to file the P.N.D.51 half-year return in August. What should I do and what is the fine?

If you forgot to file the P.N.D.51 by the end-of-August deadline, prepare the net profit estimate and file an additional return with payment of the tax immediately, before filing the annual P.N.D.50. You will have to pay a criminal fine of up to 2,000 baht [1] and a surcharge of 1.5% per month on the tax due under Section 27 [2]. Filing before the end of the accounting period exempts you from the 20% surcharge under Section 67 ter [3].

Expense documents are lost and there is no original tax invoice. Can they still be booked?

They can be recorded as a tax expense if there is evidence that the payment was actually made for the benefit of the business, such as a payment voucher with a copy of the payee's ID card, a transfer slip, or a certificate in lieu of a receipt. However, the input tax cannot be offset in the VAT system unless you contact the seller to issue a substitute tax invoice under Section 86/12 of the Revenue Code [4].

I have not filed P.P.30 and withholding tax for many months. How do I file the backlog without triggering a strict audit?

To file several months of overdue P.P.30 and withholding tax returns correctly, prepare complete input and output tax reports and reconcile the bank statements, then file through the e-Filing system and pay the tax and surcharges voluntarily before receiving a warning letter from the officer, so that you qualify for the penalty reduction under Section 89/1 of the Revenue Code [4].

Conclusion

An accounting backlog that has built up through Q3 can be resolved systematically: start with a bank reconciliation to confirm income and expenses, sort the monthly tax documents, resolve payments without receipts, and file the overdue tax returns to stop the 1.5% monthly surcharge, together with a correct additional P.N.D.51 before the end of the accounting period. This reduces the risk of a retroactive tax assessment and saves a significant amount in fines.

In addition, retail shops or service businesses that want systematic control of sales documents and inventory from the source can adopt a point-of-sale (POS) program and an inventory management system. Clients who use monthly accounting services with the accounting firm are entitled to use the POS program free of charge while on the monthly accounting service, which makes period-end bookkeeping accurate and immediately verifiable.

References

  1. Revenue Department. (2026). Revenue Code Section 35, criminal fines for failure to file a tax return. Retrieved from https://www.rd.go.th
  2. Revenue Department. (2026). Revenue Code Section 27, surcharge for late payment of tax. Retrieved from https://www.rd.go.th
  3. Revenue Department. (1999). Revenue Department Instruction No. Tor.Por. 81/2542 and Revenue Code Sections 67 bis and 67 ter. Retrieved from https://www.rd.go.th
  4. Revenue Department. (2026). Revenue Code Sections 89 and 89/1, VAT penalties and penalty reductions. Retrieved from https://www.rd.go.th
  5. Department of Business Development. (2022). Accounting Act of 2000 and Act on Pinai Fines of 2022. Retrieved from https://www.dbd.go.th

If you are struggling with an accounting backlog, documents that do not add up, or feel uneasy about the coordination with your current accounting firm, you can switch to a professional team to take over and set up a new system smoothly. An initial consultation is free of charge. Contact YMTD Accounting Partner Chiang Mai, Tel. 081-542-3019, or add LINE: @premwittawat

Note: this article is general information for educational purposes, based on the laws and notifications in force as at 6 October 2026, which may change. Every business has different details; consult a specialist before making decisions.