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HMRC internal manual

National Insurance Manual

Class 2 National Insurance contributions: Small Earnings Exception: How Net Profit is calculated: Admissible expenses: Adjustments for private or domestic expenses

The apportionment of expenditure between business and private or domestic use depends on the particular circumstances of the case. Where premises are shared, the relative areas used for business may be a guide to the proportion allowable for mortgage interest, rent, rates, lighting and heating.

For motor expenses, the division should be according to the proportion between business and total mileage.