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The roof pitch in the side return is dictated by the right to light of the adjacent properties windows. Hence why you’ll rarely get flat roofs on side returns. I don’t think it makes a lot of sense in that specific scenario, but it’s a case of a planning guideline being designed for one scenario and having to be applied to all.
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Most relevant things to look at are the permitted development guideline. pdf.- to decide if you need full planning - if so - the local council's SPG (residential design guide).is next. Different councils have different rules for boundary wall height. some as low as 2.6m. "Rights to light" is separate to planning legislation - its civil law i think. The borough will have a standard for what's acceptable on a side return they are considering loss of outlook, privacy, amenity - generally for habitable rooms only.
Now franticly googling side returns, would be game changing for our house, the back end is very Victorian and disjointed from the garden