As a sole trader you must be a genuinely self-employed worker (i.e. not an employee). This does not mean that you cannot also do other work as an employee, but the work you do for your business must be done on a self-employed basis. If HMRC think that you are not self-employed but are an employee at your client there are tax implications.
Our Employment Status App in Connect has been developed to provide an overview of how to ensure that you are working as a genuine self-employed worker and to help you recognise which factors are considered to be of relevance. It also allows you to inform us of your particular circumstances so we can confirm that you are genuinely self-employed or whether there are any elements of your working arrangement that may lead to HMRC considering you to be an employee.