What are padstones (and who specifies them)?
A padstone is a strong concrete or high-strength block placed under the end of a beam to spread its load into the wall below.
The structural engineer specifies the padstone size and material as part of the bearing design. I show the padstone and the beam's bearing on the drawings - I don't size it; that's the engineer's calculation.
If your calcs mention a padstone, that detail carries through onto the setting-out drawing.
Steel drawings for extensions (from your calcs)
Structural sizing, spans and load calculations must be carried out by a qualified structural engineer. These pages help you plan the drawings you need - they are information only and are not structural engineering advice.