How does Marriage Allowance work?
The low-earning partner must apply to the HM Revenue and Customs to request any unused personal allowance be transferred to their spouse.
Those earning less than this personal allowance amount can transfer a maximum of £1,250.
If you earn less than £11,250, which is the personal allowance, minus £1,250, you can do this without being liable to pay any tax.
In 2020/2021 those earning above £11,250 but below £12,500 can still transfer £1,250, but they will liable to pay tax on any income in excess of £11,250.
Their partner still makes a tax savings of £250, but the extra tax they pay reduces the overall level of savings made by the couple.