What a Contractor Invoice Must Include — Legally and Practically

Commercial clients bounce invoices for missing purchase order numbers. Accountants bounce them for missing VAT breakdowns. Property managers bounce them because the job address isn't on them. And every bounce restarts the payment clock from zero — the 30 days begins again when the corrected invoice arrives, not when the work was done. Getting the invoice right first time isn't admin perfectionism; it's the fastest payment-speed upgrade available.

The legal minimums (UK)

The legal minimums for a UK invoice: the word 'invoice', a unique sequential number, your business name and address, the client's name and address, the issue date, a clear description of what's being charged for, and the amounts. If you trade as a limited company, add the registered company name, number and registered office — it's a Companies Act requirement on business paperwork, and its absence looks amateur to anyone who knows.

VAT invoices and the reverse charge

VAT-registered? The bar rises: your VAT number, the net amount, the VAT amount and rate (per line if rates differ), and the gross total. On construction work between VAT- and CIS-registered businesses, the domestic reverse charge often applies instead — no VAT charged, and mandatory wording on the invoice saying the customer accounts for it. An invoice that charges VAT when the reverse charge applies isn't generous; it's wrong, and unwinding it wastes everyone's month.

US invoices: PO numbers rule

In the US, invoicing is barely regulated by comparison, but practice fills the gap: sales tax itemised where your state applies it to your work, your EIN handy because commercial clients will request a W-9 to issue 1099s, and — above all — the client's PO number if they gave you one. In corporate accounts-payable systems, an invoice without the PO number doesn't get argued with; it gets lost.

What the law doesn't require but cash flow does

Now the lines the law doesn't require but your cash flow does. A due date written as a calendar date — 'Payment due by 22 May 2026', not 'net 14', which forces the client to do arithmetic they won't do. Your bank details with the invoice number as the payment reference, so money arrives labelled. The job address, not just the billing address — a landlord with six properties needs to know which roof this was. The quote reference, tying the bill to the price they already agreed. And any deposit or retention shown as visible deduction lines, so the total reconciles at a glance.

Numbering discipline

Numbering discipline is worth thirty seconds of thought: sequential, never reused, no gaps that look like deleted invoices. If you're ever inspected, a clean unbroken number sequence is quiet evidence of a business that keeps records; a sequence that jumps from 104 to 121 invites questions you don't want. Software does this for you automatically — one of the better arguments for retiring the spreadsheet.

Descriptions that pre-answer questions

Description quality decides whether the invoice raises questions. 'Labour — building works, £1,900' is a question waiting to be asked, and questions delay payment. 'Remove existing fence, supply and install 12 panels of closeboard fencing inc. posts and concrete — £1,900' pre-answers the dispute before it forms. Specific descriptions also protect you a year later, when memories of what was agreed have softened on both sides.

Finish with the one-glance test: hand your invoice to someone who knows nothing about the job, and time how long they take to find who it's from, what it's for, how much, by when, and how to pay. Five seconds or less on all five, and your template is done. Anything slower, fix the template once — every invoice after that inherits the fix.