Where any part of a dwelling-house, being a dwelling-house to which one of the three last preceding sections applies, has been specially constructed or adapted for use for the purposes of any trade, business or profession, the gross value thereof, as ascertained under the relevant preceding provisions of this Part of this Act, shall be increased by such amount, if any, as may be just, having regard to any extra annual value for the time being conferred on the dwelling-house by the said special construction or adaptation thereof :

Provided that, where the dwelling-house is a flat or maisonette, any special construction or adaptation thereof shall only be taken into account under this section if and in so far as it has not already affected the apportionment required to be made of ;the hypothetical 1938 cost, of construction, and the hypothetical 1938 site cost, of the building of which the flat or maisonette forms part.