The situation
This is an illustrative example, not a real customer. Consider a plumber employed by a firm Monday to Thursday and working on his own jobs on Fridays and weekends. His employment income is taxed through PAYE, but his self-employment profit, around £9,000 a year after parts and fuel, has never been declared properly. He files a Self Assessment return and needs both income streams on it.
Getting organised
His employment side is straightforward: a P60 and figures for any taxable benefits. The self-employment side needed new habits. He photographs every parts receipt at the merchant, logs each Friday job with its income in a simple record, and keeps fuel receipts with a business-use note for the van.
His expense claims are the ordinary ones: materials and parts for each job, van running costs on the business share, tool purchases through capital allowances, public liability insurance, and phone costs on a sensible business fraction.
The result
With both income sources in one place, the return computes the tax on his self-employment profit after his employment allowances have done their work, and nothing is double counted. His payment on account position is flagged in advance, so January holds no surprises.
The lesson generalises: mixed income is not harder tax, it is just two sets of records that need to stay separate until they meet on one return.