I agree with Adrian that you should refer to the accountant if you're not familiar with CIS. Whether or not Brian operates under the CIS regulations isn't a choice - he's doing construction-related work, so he needs to be registered for CIS.
It sounds like the ltd company might already be registered as a subcontractor for CIS, since Avecco are deducting tax at source. Brian can apply for gross payment status for this, but there are various criteria the ltd company has to meet in order to do that - see http://www.hmrc.gov.uk/cis/subcontractors/advice-pay.htm#2
Since Brian is also employing someone to do construction-related work, he must also register as a contractor, and contact HMRC to verify any subcontractors he takes on - he should have done this before employing his mate. HMRC would then have told him the rate at which he has to deduct tax. It's not the case that Brian can choose to make his mate entirely responsible for his own tax (unless HMRC tell him to pay his mate gross).
Another issue Brian might need to consider is that he might not be able to choose his mate's employment status - if this is the only work that his mate does, he is likely to be classed as an employee rather than a subcontractor, and therefore payable under PAYE rather than CIS.
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