Where a married couple or civil partnership have jointly owned assets, the legislation in Income Taxes Act 2007 ss 836 and 837 automatically allocates income on a 50:50 basis, irrespective of the actual ownership (there are some exceptions).
If the actual ownership is on a basis different to 50:50, then a joint declaration can be made to HMRC specifying with evidence the underlying beneficial interest of the asset and thereby the income.
The form must be submitted to HMRC within 60 days of the couple making the election to change the income split to reflect actual ownership.
It is important to note that once a declaration has been made to HMRC it is to be applied indefinitely and can’t be reversed. A couple cannot submit a Form 17 in 2018/19 to get their jointly held property income taxed 3:1 and then decide in 2021/22 to ‘waive’ the form so that they revert to being taxed equally.
The only way to amend the split once Form 17 has been submitted is to actually change the underlying ownership between the couple. This firstly has the effect of resetting the arrangement to 50:50 default automatically.