Interim Invoices: How to Bill for Ongoing Projects

Interim Invoicing: Getting Paid As You Go

On month-long or year-long projects, waiting until completion to invoice hurts cash flow. Interim invoicing lets you bill for work completed to date, with final invoice at the end.

Why Interim Invoices Matter

• Cash flow — You get paid monthly, not at the end
• Risk mitigation — If client cancels, you've already been paid for work done
• Professional standard — Shows you're organized and professional

Structure: Monthly Interim + Final Invoice

May 2026 (Interim 1): £2,500 for Month 1 work
June 2026 (Interim 2): £2,500 for Month 2 work
July 2026 (Interim 3): £2,500 for Month 3 work
August 2026 (Final): £1,500 + final adjustments

Total Project: £9,000

Invoice Format

INTERIM INVOICE

Your Company Ltd
123 Business Lane
London, EC1A 1BB

Project: Website Redesign for ABC Ltd
Contract Value: £9,000
Invoice Period: May 2026

Services Delivered
─────────────────────────
Design and wireframing (May): £2,500
(Represents 27.8% of total project value)

Payment Terms: Net 14
Total Due: £2,500

This is Interim Invoice 1 of 4 for the above project.

Key Points

• Label clearly — "Interim Invoice" or "Progress Invoice"
• Reference the contract — Show the overall project value
• Show percentage — How much of the project is complete
• Use unique numbers — Interim #1, Interim #2, etc. (or same invoice series with -INT suffix)
• VAT handling — Charge VAT on each interim invoice normally

Retention: Holdback Amount

Some contracts include a "retention" — a percentage withheld until project completion:

• Monthly invoice: £2,500
• Less 10% retention: -£250
• You receive: £2,250 now, £250 held until final invoice paid

FAQ

How often should I invoice?

Typically monthly for ongoing projects. Weekly for very short sprints (1-2 weeks).

What if the project changes scope mid-way?

Issue a revised contract or variation order with the client. Update future interim invoice amounts accordingly.