Couples who jointly own rental property are usually taxed 50:50, even if they own different shares. But if you're married or in a civil partnership, Form 17 lets you split income based on actual ownership—provided you meet HMRC's rules.

The standard tax treatment for couples living together, whether married or in a civil partnership, is that property income held jointly is split 50:50, regardless of the actual ownership proportion.

However, if the ownership is unequal and the couple wishes to have the income taxed in line with their respective shares, they must notify HMRC and provide evidence of the unequal beneficial interests in the property. This is done by submitting Form 17, which declares the beneficial interests in joint property and income.

A Form 17 declaration can only be made by spouses or civil partners living together who own property in unequal shares, with the income allocated in proportion to these shares. Couples who are separated or in other types of relationships are not eligible to submit a Form 17 declaration.

The declaration is only valid if both partners agree. If one partner disagrees, the income will continue to be split 50:50, regardless of the ownership structure.

Once submitted, a Form 17 declaration remains in effect until there is a change in the couple's status, such as separation or divorce, or a change in the ownership structure. If either of these occurs, the 50:50 income split will be reinstated.

There are specific situations in which Form 17 cannot be used, such as when spouses or civil partners own property as beneficial joint tenants, income from shares in a close company or for partnership income.

In cases where property is owned in unequal shares, submitting a Form 17 declaration can offer tax benefits under certain circumstances.