You are liable for capital gains tax on any profit you make between what you bought it for and what you eventually sell it for, at the time when you eventually sell it. However you can offset any gain against all the expenditure you have made on improving it (items that have been incorporated into the fabric of the structure).
So you should keep each and every one of these receipts.
You should also, of course, be declaring the income from letting as income on your annual tax return form.