Tax Advisor

Does an employer "have to" register for PAYE scheme with HMRC if the staff is being paid below the Secondary threshold of £9100?

Hello, Client received letter for Let Property Campaign who now wants to declare the rental income. A UTR number has been issued as HMRC was informed and this has now opened the window of tax returns within the HMRC portal for the past 4 years. We are aware that HMRC system only allows 2 years tax returns but once notified of filing it does give the option to submit tax years before that too albeit this is not known to the HMRC staff we spoke over the phone. The question is that if we submit the tax returns to HMRC, for all the concerned years, from HMRC portal without using the Let Property Campaign (LPC) form, would that suffice? If yes, then who will calculate the penalties and interest. Furthermore, is filling of the LPC form compulsory to declare under the LPC? Looking forward to hear from you. Thank you.