Foundations - Clear site or cut trench first

[tvrulesme]

I'm building a small extention which will be a link between the main house and a detached garage. The garage sits ~45cm higher than the house so the foundations have been designed to be stepped.

 

When it comes to building, does it make sense to cut the trench directly out of the concrete patio in the picture below and dig down or to completely clear the site and then dig the foundations?

 

My thinking is that by removing less patio it will have less chance of collapsing but would be grateful of opinions.

 

Blue lines below show the path of the inside walls of the extension

 

 

[Russell griffiths]

Smash it all out down to a flat level area. 

[Mr Punter]

+1

[saveasteading]

+2

There are unknowns so get rid of it.

[tvrulesme]

2 minutes ago, saveasteading said:

+2

There are unknowns so get rid of it.

Yeah that makes perfect sense. Thank you. I do fortunately know exactly where all drainage and services go but garage foundations are an unknown so this will at least give me a view on that