We need piles because we're on 'made ground'.
We've chosen to use stone columns instead of steel piles. So far so good.
We need to satisfy SUDS requirements.
So, what's wrong with popping (say) four more closely grouped stone columns in our garden to act as a soakaway? No need to compact the stone to 150kN, but we could easily (20 minutes) pop four stone columns in for a few extra quid, and have a soak away.
Too simple innit?
What have I missed?