Max out your loft allowance
While the old volume calculations are long gone in Class A, as mentioned already, the one place they remain is with lofts under Class B.As long as you stay no higher than the highest part of the roof, the rules allow you to build additional volume of up to 40 cu m on terraced houses and up to 50 cu m on semi-detached and detached houses. These allowances are pretty generous in all honesty and most small houses would struggle to get near to the maximum anyway, but you can often come up against the limit on larger houses.
This image shows one house, on the right, that has been extended with a fairly standard box dormer, probably well within the volume limit. The owners of the house on the left, however, have really gone for it, extending over the rear part of the house, too, getting an additional room and probably knocking close to the maximum.