Readers’ forum: Tax and NI treatment of additional income

I have a client with self-employment income regularly in excess of £100 000. She has recently accepted a part-time office generating an annual income of about £15 000 (with only a negligible impact on her professional profits). I believe that this additional income falls within terms of ITTOIA 2005 s 16C although there might be different views as to whether £15 000 is considered insubstantial.

Assuming that s 16C applies how is it possible to get an NT code applied to the office income? What happens to any PAYE that is deducted in the meantime? I assume that travel expenses will then be subject to the ITTOIA rules rather than those under ITEPA. Is that correct?

In addition what happens to National Insurance? Can the two sources be similarly merged so as to avoid primary contributions being paid? How do the contributions caps operate in such...