Is misinterpretation a reasonable excuse, and alternative fines query

I was told that if your total income is under £10,000 and your income after allowable deductions is £2,500 then it is not necessary to fill a self assessment. I now realise that I have accidentally misinterpreted this and now realise that I need to make a worldwide declaration for foreign income from property. Firstly, would HMRC view a genuine misinterpretation as an excuse for not filling a declaration, or would they view this as careless behaviour? Secondly, Everything has been properly declared and paid abroad, as it would have been in this country if I had understood my obligations correctly, and due to the double taxation agreement I believe that no additional tax is due in the UK. I have read that any tax due must be paid and the appropriate fine would be double the tax paid. Since no tax is due so doubling this is not possible, would there be a different fine for the accidental non declaration? I am very worried that I may miss something else and get into more trouble.

Hi,
 

Thank you for your question. Unfortunately, I would only be able to offer general advice on the matter as numerous aspects would need to be considered regarding your specific query. Under normal circumstances, failure to submit a Self-Assessment Tax Return would result in penalties staggered over a series of periods. Please refer to the attached guidance which outlines the penalties process and reasonable excuse guidance, as well as if any changes are required once you have submitted your Tax Return follow : www.gov.uk/self-assessment-tax-returns/penalties . If any penalties are inserted onto your record, please see the Appeals process also, : www.gov.uk/guidance/check-when-to-appeal-a-self-assessment-penalty-for-late-filing-or-late-payment . Thanks