UK Tax Questions? Ask a UK Tax Advisor for answers ASAP
Thanks for your question - I am Sam and I am one of the Uk tax experts here on Just Answer.
I cannot advise whether you should add 20% to your cost price - as this is up to you - and whether this is something you usually do to make a further profit but you do not undertake this at all just for the purposes of VAT - as you are not registered for VAT
But I can advise that you should mention nothing about VAT on your invoice (as you are not VAT registered) and should just lay out your invoice as your usually do - Labour plus Materials = Total invoice cost (and many mark up the materials)
If then the company for whom you will carry out this work feel it falls within the CIS scheme then they will tax you on the labour element only - and then should provide you with a statement of deduction (at least every month) for you to then take any tax suffered into account for your year end self assessment.
Let me know if you require any further assistance