Hi,
You may incur expenses for the purposes of a rental business before that business starts. If this occurs, you may be able to claim a deduction once the letting begins. In order to be allowable, the expenditure must be incurred wholly and exclusively for the purposes of the rental business and not of a capital nature. You can see the guidance here:
PIM2505 - Beginning and end of a rental business: commencement
PIM2030 - Deductions: repairs: is it capital?
Depending on the type of expense incurred, these may not be allowable, however, when sold you may be able to offset these against any capital gains liability.
Thank you.