Smalllee Lee

When the spouse are entitled to apply the split year for tax year, if only 1 spouse filled in the form (SA109), another one is not going to fill in (because his income for full tax year less than GBP12,570 (covering to Overseas and UK part), to minimise the paperwork and save cost using 3rd party recognised software for online reporting). Will affect them to transfer 10% allowance to another using Marriage Allowance? Thanks

If I fulfil more than 1 case of split year treatment, should I put the applicable cases into the SA109 Form Box 40? Will HMRC select the favour case to the tax payer (which is the lowest tax burden)? Or the tax payer needs to determine which case to be applied for the relevant tax year? Thank you.