Foundations carry more load
Foundation design depends on ground conditions, nearby trees, drains, depth of existing foundations and the proposed structure. The fact that a neighbouring single-storey extension uses a particular trench depth does not make that depth suitable for yours.
Upper-floor layout can drive the structure below
Bedroom walls, bathrooms and stair positions affect beams and joists at first-floor level. Designing the upper floor after the ground floor has been fixed can create awkward supports.
Planning impact is greater
Upper-storey work can affect overlooking, outlook and the apparent bulk beside a boundary. In practice, many two-storey proposals need a planning application even where smaller works might fall within permitted-development rights.
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