i am VAT registered and I need to invoice a company that is not vat registered and does not charge vat for its services. Do I need to add vat to my invoice when dealing with this company?
Short answer, yes you charge VAT. If you are VAT registered, you are responsible for charging VAT, regardless of the VAT status of your customer.
Full guidance here
I would suggect that you include "+ VAT" in all of your agreements and contracts to prevent disputes.
If the customer is inside the UK, you add on 20% VAT to the invoice. (Even if they are VAT registered.)
If the customer is inside the European Union and not VAT registered, you apply UK 20% VAT to the invoice. (Unless you exceed the local threshold, in which case you need to be VAT registered in that member state and apply local VAT.)
If the customer is inside the European Union and VAT registered, you zero-rate VAT and state 'This supply is subject to the reverse charge'.
If the customer is outside the European Union, you do not charge VAT because it is outside the scope. Note there is a difference between 0% VAT and no VAT.