Registering as a self-employed subcontractor in construction
Registration under CIS is in addition to registration as self-employed for self-assessment, not instead of registering as self-employed. This means that there are two separate registrations, however both can be done at the same time. When you contact HMRC to register as self-employed you need to ensure that you are also registered as a CIS subcontractor.
In most cases you can register as self-employed by calling the Newly Self-employed Helpline on 0300 200 3504. If you are already registered as self-employed, but need to register under the CIS scheme, you should contact the CIS Helpline – 0300 200 3210.
You will need your self assessment Unique Tax Reference (UTR) number when you start work. The contractor for whom you are working will ask you for your UTR, before you are first paid, in order to determine which tax deduction rate to use.
The UTR is issued when you are first set up under self assessment to complete a tax return. If you have not previously been required to prepare a tax return, you will be given a UTR when you register as self-employed.
There is guidance on registration and other obligations for subcontractors at https://www.gov.uk/what-you-must-do-as-a-cis-subcontractor.