Expenditure incurred on the purchase of patent rights before 1 April 1986 is written off on a straight-line basis. The writing down period is normally 17 years and so the annual allowance is one-seventeenth of the expenditure. The writing down period is less than 17 years in these two cases:
There is a balancing charge where patent rights are sold in
whole or in part and the net capital proceeds of sale exceed the
capital expenditure remaining unallowed. The balancing charge is
equal to the excess.
There is a balancing allowance where patent rights lapse or
are sold and there is capital expenditure remaining unallowed which
exceeds the net proceeds of sale. The balancing allowance is the
amount of that excess.