Hi,
Generally speaking, expenditure on repairs and maintenance is deductible from rental income, while any improvement costs, for example any extensions, fall into the category of capital expenditure. You can see detailed guidance here:
PIM2030 - Deductions: repairs: is it capital?
If expendiiture exceeds rental income in a given year, the resulting loss can be carried forward and set off against rental profits in the following year. You can see more information here:
PIM4210 - Losses: setting losses against future profits
A definitive guide to allowable rental deductions can be seen here:
PIM1900 - Deductions
Thank you.