The opening into the old house matters
Removing an external wall changes the structure and may alter fire separation, ventilation and energy performance. The beam above the opening is only one part of the Building Regulations work.
The floor and walls need continuous thermal detailing
Insulation performs badly where junctions leave cold bridges. Floor edges, lintels, roof-to-wall junctions and window reveals deserve as much attention as the middle of a wall.
Drainage must remain maintainable
Building over or close to existing drains may require changes to the route or access arrangements. Drawings should show known drainage before foundations are finalised.
Completion paperwork matters later
Keep approved drawings, structural calculations, inspection records and completion certificates together. They are commonly requested when the house is sold.