Working out gains when selling a rental property

Gov.UK says that you can deduct costs of buying, selling or improving your property from your gain. These include estate agents and solicitors fees, which is simple. It also says "costs of improvement works, for example for an extension (normal maintenance costs, such as decorating, do not count)" My question is. is there a timescale attached to this? Can you deduct the cost of, for example, and extension you built 12 years ago? Or does it all have to be within a certain time period before the sale?

Hi, It relates to any period during the time of ownership. Thank you