Hi,
When the joint owners of a property are husband and wife, profits and losses are treated as arising to them in equal shares unless both parties have signed a declaration stating their beneficial interests in both the property and the income arising from it differ. The declaration must be made on form 17, both parties must sign the decalaration or the standard 50:50 split will apply. The following guidance provides further information on evidence of beneficial ownership, such as a declaration of trust should be submitted along with the form 17.
TSEM9842 - Property held jointly by married couples or civil partners: Form 17 rule: introduction
Expenses claimed will be apportioned in line with the agreed beneficial interests in the income.
You can see information regarding the expenses incurred at the start of a rental business here:
PIM2505 - Beginning and end of a rental business: commencement
Thank you.