A wasting asset is defined as any asset which has a predictable life which does not exceed 50 years. As the name suggests, a wasting asset is likely to become less valuable over its predictable life. At the end of that life, it has only a scrap or residual value.
It is necessary to predict the life of the asset at the time of its acquisition by the person who has acquired it. In some cases, this may be obvious. A lease of land may be for a specific period. That period is then the predictable life of the lease as far as the lessee is concerned.
In other cases, it will not be obvious and you have to estimate the predictable life. In many cases, you will only have to do this after the asset has been disposed of. However, you still have to look at the position as it was when the asset was originally acquired by the person making the disposal.
Residual or Scrap Value
The residual or scrap value of a wasting asset is the amount it will be worth at the end of its predictable life. Again, this has to be estimated by reference to the position as it was when the asset was acquired by the person making the disposal
A person who owns a wasting asset may incur additional expenditure which enhances the value of the asset. This will not affect the predictable life of the asset but may alter its scrap or residual value.
You will find the rules which tell you about Capital Gains Tax and the disposal of wasting assets in TCGA92/S44. CG76706+ tell you how these operate.