The old rear wall becomes structural work
If most of the rear wall is removed, the masonry above it needs permanent support. That may be a steel beam, more than one beam, or a frame where side supports are limited.
Existing drains can dictate the footprint
Soil pipes, manholes and combined drains are often found close to the back of older houses. A drawing that ignores them can change quickly once excavation begins.
Do not lose useful garden for poor internal space
An extension can be deep yet still leave the old middle room dark. Rooflights, a courtyard, a shallower extension or a better opening between rooms can sometimes produce a better result than simply taking the maximum depth available.
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