Hi.Take a look here for information on personal allowances. The allowance an individual can have is dependent on their age and income and can be £10,000, £10,500 or £10,660.I'm not sure what you mean when you say ""it is safe to declare her rental income because her income still falls below the threshold of personal allowance". It isn't a choice I'm afraid. If the net of expenses rental income is £2,500 or more or is £10,000 or more before expenses then a self-assessment tax return must be submitted to the tax office. There is a guide here. The lady you are referring to can register for self-assessment here.I hope this helps but let me know if you have any further questions.