Establish the original roof
'Original' means the building as it stood on 1 July 1948 if it existed then, or as first built if constructed later. It does not mean the roof when you bought the property or the roof immediately before the proposed work. [1]
Use the survey and available historic drawings to distinguish the original roof from later alterations. Identify the parts that will remain, be removed or be enlarged. Earlier roof-space additions count even where they were built with express planning permission rather than permitted development. [1]
A 2024 Planning Inspectorate appeal in Merton illustrates the risk: the assessment included roof volume associated with a later side extension, rather than treating only the latest dormer and hip-to-gable proposal as relevant. It is a useful example of why the original baseline matters, not a substitute for assessing your own building. [2]
Calculate the resulting increase
A practical calculation package should:
- Draw or model the original roof envelope.
- Draw the complete resulting roof, including retained previous enlargements.
- Calculate the external volumes using consistent dimensions.
- Show the increase over the original roof and compare it with the appropriate allowance. [1] [2]
Planning Portal provides calculators for several common roof forms, including flat-roof dormers and hipped roofs. A calculator can assist with geometry, but it does not decide which roof parts belong in the legal assessment. Intersecting roofs need particular care to avoid missing or double-counting space. [3] [1]
A worked example
Suppose an eligible terraced house has 12 cubic metres of retained enlargement above its original roof, and the proposed dormer adds another 24 cubic metres without overlapping that enlargement. The resulting increase would be 36 cubic metres, leaving 4 cubic metres within the 40-cubic-metre ceiling. These are illustrative numbers, not measurements for a real property. The cumulative rule comes from Class B. [1]
The same house would not receive a new 40-cubic-metre allowance when it changes owner. Nor does calling one part a hip-to-gable and another a dormer give separate allowances. [1]
Check the external details
The Planning Inspectorate decision confirms that cubic content is measured externally. A calculation based only on the clear space inside the room is therefore unsuitable. Give the person preparing the calculation the actual proposed roof build-up and external dimensions, not just a furniture plan. [2]
Volume is only one test
A proposal below the volume limit can still fail because of its height, position, eaves treatment, windows, materials or site designation. Keep a dimensioned volume schedule with the rest of the Class B assessment and, where a lawful development certificate is sought, ensure it matches the submitted drawings. [1] [4]