The difficult part is often where new meets old
The outer walls of a small rear extension are usually straightforward. More thought is needed at the opening into the existing house, the roof-to-wall junction, existing drains and the finished floor level. Those details can determine how much steel, excavation and internal alteration is required.
Room size affects more than floor area
A wide opening into the existing house may need a substantial beam or goalpost frame. Large rooflights can change roof structure. Wide sliding or folding doors need adequate support above and a threshold detail that still keeps water outside.
Check the level from garden to house
A low garden, high patio or sloping site can complicate damp-proof levels and steps. The extension floor should not be designed in isolation from the existing floor and external ground.
A simple shape usually helps
Every additional corner, valley, parapet or awkward roof junction adds labour and another place that has to remain weather-tight. A plain rectangle can still produce a good room and is easier to build cleanly.
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