CIS deduction checker
A small system for any contractor paying subcontractors under the Construction Industry Scheme: verification kept in date, every invoice split into what is deducted from and what is not, the reverse charge checked before anything is paid, and a monthly close that produces the return, the statements and the evidence from one frozen set of numbers. Seven posts on the same system, one diagram at a time, with a cost breakdown and an engineering reference at the end.
- 01
A CIS deduction checker on AWS for a few dollars a month
The whole system on one page — verification, the invoice read, the deduction and the monthly close — and why the materials line rather than the rate is where contractors get caught.
- 02
Verifying a subcontractor before the first payment
Deciding per payment whether a verification is still in date, recording the treatment, verification number and date HMRC returned, and handling unmatched results and status change notices.
- 03
Labour, materials, and the line between them
Splitting a subcontractor invoice into the deduction base and what comes out of it — VAT, materials, plant hire, consumables and fuel — with the evidence for each materials figure recorded at the time.
- 04
The reverse charge, and the VAT line that should not be there
Checking each inbound invoice against the domestic reverse charge conditions, recording which way the end-user notices run, and holding invoices that charge VAT they should not.
- 05
Closing the tax month by the nineteenth
Freezing a tax month, building the return and the payment and deduction statements from the same rows, paying HMRC, and keeping the trail from any deduction back to a receipt.
- 06
What the CIS deduction checker costs
About $3 a month. One Bedrock read per invoice is the only line that grows; the queue, the table, the mail and the storage are rounding errors. Plus the three ways the bill could surprise you.
- 07
Engineering reference: the CIS deduction checker architecture
Same system, drawn purely for engineers. Service names, region, Lambda inventory, IAM scopes, the schemas and the exact model id.
Frequently asked questions
- What is the CIS deduction actually taken from?
- The payment excluding VAT, less what the subcontractor actually paid for materials, consumable stores, plant hire, fuel other than fuel for travelling, and the manufacture or prefabrication of materials used in the work. Labour, travel and profit stay in the base.
- What if a subcontractor has no receipts for their materials?
- HMRC’s guidance lets you estimate the cost, but it also says the contractor must check the materials figure is not overstated. Where it looks excessive, HMRC can seek the under-deduction from the contractor.
- Do we have to verify a subcontractor every time we pay them?
- No. You do not need to verify again if they were included on one of your CIS returns in the current tax year or the previous two. Outside that window, verify before paying.
- Does the VAT reverse charge change the deduction?
- No. The deduction base always excludes VAT. The reverse charge changes who accounts for the VAT, and its most common failure is a subcontractor invoice that still charges it.
- What does it cost to run?
- A few dollars a month. See part six.