Why am I on 30% CIS instead of 20%?

The three causes

1. You are not registered under the scheme. Registration is separate from having a UTR and separate from being registered as self-employed. Plenty of people who have filed tax returns for years have never registered for CIS.
2. The contractor could not verify you. Verification matches the details the contractor holds against HMRC's record. A middle name, a trading name used instead of the legal name, an old UTR, a company number that does not match — any of these can produce an unmatched result, and an unmatched result is a higher-rate deduction.
3. The contractor did not verify you at all. Where no verification was attempted, the higher rate is what the rules require them to apply.

How to find out which it is

The payment and deduction statement is the evidence. Where a deduction has been made at the higher rate the contractor must show the verification reference, and that reference carries a letter on the end of it. The presence of that suffixed reference tells you the contractor did verify and got an unmatched result. Its absence suggests no verification was carried out.

Once you know which it is, the fix is different: register, correct the details HMRC holds, or ask the contractor to verify again.

What it costs while it runs

Ten extra points of your labour, held until after the tax year ends. On £48,000 of labour-only turnover that is £4,800 of your money sitting with HMRC for up to twenty months. None of it is extra tax — it all comes back — but it comes back late.

If you are consistently in a repayment position, gross payment status stops the deduction happening at all.

This question is part of CIS for subcontractors, which covers the whole area.