Supplementary Invoice Template
A supplementary invoice charges an extra amount on a supply you have already invoiced: work or goods left off the original invoice, a price increase agreed afterwards, or tax undercharged by mistake. It leaves the original invoice in place and adds a second document that quotes the original number and date, explains the reason and bills only the difference plus tax on it. It works like a debit note issued by the supplier, and in India's GST law a debit note expressly includes a supplementary invoice. It is the opposite of a credit note, which reduces an amount already invoiced. This editable HTML template has fields for the original invoice reference, the reason, the original and revised amounts, the increase, the tax on the increase and the amount now due. Fill it in your browser, then print it or save it as PDF.
Preview this editable document
Download the HTML file to edit its fields, then print or save as PDF. Totals are entered manually.
What This Template Includes
- Document title: Supplementary invoice
- Supplementary invoice number
- Issue date
- Due date
- Original invoice number and date
- Reason for the additional charge
- Date the increase was agreed (if a price change)
- Description of the goods or services the increase relates to
- Original amount, revised amount and increase
- Tax rate and tax on the increase
- Amount due on this invoice
- Your tax registration number, if registered
- Payment instructions
How to Create Your Supplementary Invoice
- 01
Describe your work
Type a plain English description of the services you provided, the client, and the amount.
- 02
AI generates your invoice
InvoiceQuickly's AI drafts the client, line items, dates, and totals for you to review before saving.
- 03
Review, download, and send
Check every detail, download the PDF, or use Pro to email the PDF directly to your client.
Recommended Payment Terms
Use the same payment terms as the original invoice unless the contract says otherwise, counted from the supplementary invoice's own issue date. If the original invoice is still unpaid, you can ask for both together, but keep them as separate documents so each amount can be traced. If the extra charge comes from a contract clause, such as a price-variation or extra-hours clause, name the clause in the reason so the customer's approver can check it quickly.
Need help writing payment terms? Read our guide to invoice payment terms for best practices and templates.
Tax Information
Tax on a supplementary invoice is charged only on the increase, at the rate that applied to the original supply. In the UK, HMRC's VAT Notice 700 says a valid debit note must reflect a genuine mistake, undercharge or agreed price increase, be issued within 14 days of the increase being agreed, and show its own number and date, the supplier's name, address and VAT registration number, the customer's name and address, the number and date of the original VAT invoice, a description of the goods or services, the increase excluding VAT, and the rate and amount of VAT. If it corrects a VAT error, it should also show both the correct and incorrect VAT amounts (paragraph 19.8). In India, section 34(3) of the CGST Act requires the supplier to issue a debit note when the taxable value or tax on an invoice is found to be less than it should be, section 34(4) requires it to be declared in the return for the month in which it is issued, and the Explanation to section 34 says a debit note includes a supplementary invoice. This is general information, not tax advice; check with your tax authority or adviser.
Industry context (2026)
The term 'supplementary invoice' is most common in India, where it has been part of GST law since 2017. The text of section 34 of the Central Goods and Services Tax Act on the CBIC tax information portal, checked in October 2026, still carries the Explanation that a debit note includes a supplementary invoice; amendments since 2019 allow one debit note to cover one or more invoices for supplies in a financial year. In the UK the same document is usually called a debit note, and HMRC's VAT Notice 700 (last updated 25 June 2026) sets out what a valid debit note must show and the 14-day issue window. In payroll and staffing, providers sometimes use 'supplemental invoice' for variable items such as bonuses that are finalised after the regular invoice. In every case the principle is the same: the original invoice stays, and the extra amount gets its own document that points back to it.
Worked example
Assume a Bengaluru software firm and its client are in the same Indian state, so GST is charged as CGST 9% plus SGST 9% (18% in total) on both documents. On 1 October 2026 the firm issues tax invoice INV-0412 for September support: 160 hours at ₹2,000 = ₹3,20,000; CGST ₹28,800; SGST ₹28,800; total ₹3,77,600. On 8 October the client's project manager approves 12 further September hours that were missing from the timesheet export. The firm does not edit INV-0412. On 10 October it issues supplementary invoice DN-0009, titled 'Supplementary invoice / debit note', quoting 'Original invoice INV-0412 dated 1 October 2026; reason: 12 approved hours omitted from September timesheet'. Lines: original taxable value ₹3,20,000; revised taxable value ₹3,44,000 (172 hours at ₹2,000); increase ₹24,000; CGST 9% ₹2,160; SGST 9% ₹2,160; amount due ₹28,320, payable on the same 15-day terms as the original. Because DN-0009 is dated in October 2026, it is reported in the return for October 2026 under section 34(4) of the CGST Act, while INV-0412 stays in the period it was issued. Check: the two documents together bill 172 hours, ₹3,44,000 taxable value and ₹61,920 GST, which is 18% of ₹3,44,000.
Common mistakes to avoid
Reissuing the original invoice with a new total
Two documents with the same supply and different totals confuse the customer's records and your own. Keep the original and add a supplementary invoice for the difference, or credit the original in full and start again.
Charging tax on the full revised amount
Tax on the original amount was already charged on the original invoice. The supplementary invoice taxes only the increase: ₹24,000 at 18% in the worked example, not ₹3,44,000.
No reference to the original invoice
Without the original number and date, the customer cannot match the extra charge to the supply, and in the UK a debit note without that reference does not meet HMRC's listed details.
Sending a credit note when you meant to charge more
A credit note reduces what the customer owes. Use a supplementary invoice or debit note for undercharges and price increases.
Missing the time limit
HMRC expects a debit note within 14 days of the increase being agreed. In India the debit note is declared in the return for the month it is issued, so the issue date matters for which period the extra tax falls in.
Invoicing Tips
- 1Never edit or reissue the original invoice to add the missing amount once it has been sent. Issue a supplementary invoice for the difference and leave the original as it is.
- 2Quote the original invoice number and date on the supplementary invoice, and in the line description, so the customer can match the two.
- 3Show the original amount, the revised amount and the increase, so the reason for the extra charge is visible without opening the original invoice.
- 4Charge tax on the increase only, at the rate that applied to the original supply.
- 5Use a numbering scheme that marks supplementary invoices or debit notes clearly, either a separate prefix or your main invoice sequence, and apply it consistently.
- 6Agree the extra charge with the customer before you send it. A surprise supplementary invoice is more likely to be disputed than one that follows a confirmed email.
Related Resources
- All invoice and business document templates
- Debit note: glossary definition
- Credit notes explained: the opposite adjustment, for overcharges and returns
- Tax invoice vs regular invoice
- Invoice tax rules guide
Frequently Asked Questions
What is a supplementary invoice?
It is an additional invoice for an amount that should have been on an earlier invoice for the same supply, or for an increase agreed after it was issued. It references the original invoice and charges only the difference plus any tax on it.
Is a supplementary invoice the same as a debit note?
They do the same job. In India, the Explanation to section 34 of the CGST Act says a debit note includes a supplementary invoice. HMRC's VAT Notice 700 uses 'debit note' for an agreed increase and mentions supplementary invoices for correcting undercharged VAT. Note that in some businesses a debit note means a buyer's request for a credit, so label your document clearly.
What is the difference between a supplementary invoice and a credit note?
They move the amount in opposite directions. A supplementary invoice or debit note increases the amount the customer owes. A credit note reduces it, for example after returned goods, an overcharge or a discount.
Is it possible to amend an invoice instead?
Once an invoice has been sent, it should not be amended or replaced. Leave it in your records and issue a supplementary invoice for an undercharge or a credit note for an overcharge, each referencing the original. If you need to replace the invoice entirely, issue a credit note for the full amount and then a new invoice.
What is the difference between a supplementary invoice and a revised invoice?
A revised invoice replaces the original with a new document showing the corrected total, which can leave two live invoices for one supply if the original is not cancelled. A supplementary invoice adds the difference on top of the original, so each document stays valid and the audit trail is clear.
When does a UK debit note need to be issued?
HMRC's VAT Notice 700 (paragraph 18.2.4) says a debit note for an increase should be issued within 14 days of the increase being agreed between supplier and customer, and the VAT adjustment belongs in the period when that agreement is reached.
Can InvoiceQuickly create a supplementary invoice?
Only as an ordinary invoice. InvoiceQuickly has no debit note or supplementary invoice type, so you would create a new invoice for the difference and put the original invoice number in the line description or notes. It applies one tax percentage per invoice, does not split CGST and SGST and copies saved tax IDs to new invoice PDFs, so it is not a good fit where your tax authority requires those details. Credit notes for overcharges are available on Pro.