Hi,
Please see guidance available at
TSEM9812 - Property held jointly by married couples or civil partners: Overview: two main rules which advises- the ‘50/50 rule’ (ITA/S836) whereby most income from jointly held property is treated as split equally between the two spouses or civil partners for income tax purposes; the 50/50 rule applies unless there is a valid declaration on form 17; sections TSEM9814-9840 contain the details.
Unless a Form 17 is in place to declare unequal shares in the property, the income must be split equally between spouses.
Therefore, Rent a Room limit would also be split by 50% to £3750.
Thank you.