Groundworks come before visible progress
Setting out, excavation, drainage and foundations can consume time without creating much that looks like a room. Errors here are also difficult to correct later.
The shell needs to become weather-tight
Walls, roof, windows and doors allow internal work to proceed in better conditions. Structural openings into the old house are often delayed until the new shell can protect the building.
First fix locks in many decisions
Electrical cables, plumbing, ventilation and heating routes are installed before plastering. Moving sockets is easy on a drawing and awkward after walls are closed.
Finishes expose earlier inaccuracies
Kitchen units, tiles and skirtings make uneven walls and wrong floor levels obvious. Care during structure and plastering saves time at the end.