what's the effect of constructing a new building (bearing on isolated foontings) on the foundation (Piles which are about 20 years old) of an adjacent existing building. Foundation soil is SAND, loose to medium dense.
Rainer is probably right that the effect is minimal. However, you gave no real data and if the conditions are unfavorable, the existing building could experience differential settlement. Most good text books will indicate how to calculate the increase of stress outside a loaded area(s) and how to calculate settlement in sand for an increase of stress. If the new footings are close to the old building and the piles are short, perhaps you should invest time in performing an analysis.