This part of GOV.UK is being rebuilt – find out what beta means

HMRC internal manual

Compliance Handbook

How to do a compliance check: establishing the facts: asking for information: private bank accounts

You can require a person to produce statements for private bank/building society accounts where you believe they are reasonably required to establish the person’s correct tax position, see CH21540, or for the purpose of protecting, securing, collecting or managing revenues derived from duties of excise.

Non-business bank accounts should not be requested as a matter of course. However, where financial accounts are not based on a robust and effectively operated record keeping system which is supported by adequate and appropriate safeguards and/or include unvouched or unverified sums, it would be reasonable to request private bank details with the other records.

If you need private bank statements when dealing with IT/CT enquires follow the guidance in the Enquiry Manual at:

EM1561 - Business Taxpayers

EM1570 - Non Business Taxpayers