New client has not disclosed significant rental income - what are tax penalties with full disclosure

New client has advised that they have been earning about £20K per year for the last 4 years renting out 4 rooms in their private property under the assumption that the rent a room scheme exemption limit is per room and so covered the income. As far as I am aware it is per residence so there is a significant tax liability.

Assuming full voluntary disclosure, what are the likely tax penalties?

Thank you