Hi.There are many non-domiciled people living in the UK who don't have a national insurance number. I'm not aware that its compulsory to have one but your client would need one if he worked as an employee or on a self-employed basis. An outline of the process to obtain an NI number is here. If you need to complete a tax return, you will need to apply for your client to be issued with a UTR by completing a form SA1. I've completed and submitted self-assessment tax returns for non-domiciliaries without NI numbers in the past and have yet to run into problems but it appears that an NI number must be obtained before registering for self-assessment from the note on the SA1. I hope this helps but let me know if you have any further questions.