Spending a volume allowance well
Most permitted development limits are dimensional, which makes them constraining but simple. The Class B roof allowance is a volume, which is unusual and much more interesting, because the same entitlement can be spent in several different shapes and they are not equally useful. On a terraced house with a rear closet wing there are broadly three ways to spend it. A wide, shallow rear dormer across the main roof gives the most floor area at full height in a single room, and is the simplest to build. A narrower but deeper dormer concentrates the volume, which can suit a plan where a bathroom needs full height in a specific place. An L-shaped dormer wraps from the main rear roof around and over the closet wing roof, and although it uses volume less efficiently in pure floor-area terms, it is frequently the best outcome, because it connects two spaces and creates the circulation that lets the loft become a proper floor with a bedroom and a bathroom rather than a single large room. Which is right depends on the head height available, the position of the stair, and where the bathroom has to go for its drainage. Two arithmetic points are worth holding on to. The allowance is measured above the original roof plane, so a dormer set back from the eaves consumes less volume than one taken right down to them, which is sometimes the way to fit a scheme that would otherwise exceed the limit. And where an earlier alteration has consumed part of the allowance, the remaining volume can be too small for a useful dormer even though it is not zero — in which case the honest options are a rooflight conversion within the existing roof, or a householder application for something larger, which on an undesignated street is a perfectly reasonable thing to apply for.
