The Construction Industry Scheme (CIS) is where a construction contractor deducts, at source, a portion of the money due to their subcontractor. Whatever amount is deducted is then passed directly to HMRC which is then counted towards the tax and National Insurance of a subcontractor. This effectively pays the tax element in advance.
How much is deducted will depend on whether or not a subcontractor is registered under the CIS system. Unregistered subcontractors usually get a 30% deduction, while the registered ones get a deduction of 10% less or the standard rate of 20%. This is one of the reasons that, by the end of the financial year, the amount deducted at source often ends up being overpaid.
But a bigger reason is the fact that the deductions did not factor in the personal allowance that every taxpayer in the UK is entitled to. From £11,000, the personal allowance for the tax year 2017-2018 has been increased to £11,500.