Today I've registered as self-employed but I am not going to leave my permanent work (full time contract). At this moment I have on my payslips tax code 1000L. Should I tell to my employer that now I have been registered also as self employed? Will they change my tax code to BR?
no leave it is it is since your employed work is your main income you should allocate your personal allowance against it. Obviously you will have used your PA up during the year so any profits made from your self employment will be taxable from the first £1, so try to make a provision for this. Good luck with the new venture!