Is this the building you are talking about ?

http://maps.google.co.uk/maps?f=q&source...12,,0,8.11

Or the one on the corner or Brockley rise.

With regards to digging "under" the garage. I would suggest that whatever they do, they will demolish the existing building first, as neither are suitable to convert to sellable flats. Which will mean the digging will be open pit, rather than lots of underpinning and complications.

My true concern would be the planned height of the building, and any view it may obscure. Other than that, I cant see too many objections being listened to.