A two-storey extension adds floor area at a lower cost per square metre than a single-storey extension of the same total area, because the foundation and roof serve two floors. Total cost still depends on structural work to the existing house, the extent of internal remodelling, and finishes.
Adding a second storey to an extension usually improves value per square metre, since foundations, roof and much of the site set-up are shared across two floors. However, two-storey work more often requires planning permission, and it typically involves more structural intervention in the existing house, including new openings, floor connections and stair alterations.
Two-storey extensions also raise design issues that can carry cost: overlooking of neighbouring property, daylight to existing rooms, and matching external materials, all of which can be the subject of planning conditions.