extend Basement garage

Hi All,

I have a garage in the basement, of a block of 18 units

I believe we are under our landspace ratio required

it is located in rear corner of the block but 8 m or so from boundary.

and located under rear lawn,

it is a single garage, most others in block are tandem,

i would like to extend garage to make it a double by excavating thru my garage 

I am also a builder of over 30 years so I know whats involved in the structual side , drainage etc.

would the strata costs be prohibitive ? what would be involved ? there are 18 units , no one  would be effected , except for the duration of works.