Over site prep after foundation masonry. Why?

[flanagaj]

When we had the footings dug, the JCB guy told me to pour the concrete footings and then dig the over site down, before commencing the foundation masonry.  That made complete sense, but unfortunately, the brick layer was committed to starting on a specific date and it didn't get done.  Now I'm thinking how blooming tricky it's going to be to manoeuvre the mini excavator around and not catch any of the block work.  So unless I am missing something 'basic' here, my advice would be to dig it down first and then commence your foundation masonry.

[dpmiller]

why do you need to dig down?