Rule of Thumb/Best Practice Resource per m2

Hi all

Fairly new to the PP site so apologies if i am posting in the wrong feed.

I am looking to demonstrate to a client that the increase in scope requested without increasing the duration at the work face will cause all sorts of resourcing problems.

Problem is the only information we have at the moment is labour hours and a window of opportunity, no WBS nor sequence of any kind.

My thought is to show a levelled resource against the period of time and then reference a best pratice statement to hopefully show that the resource will be inefficient based on the size of area.

I am sure CIBSE or some other regulation body identify that 1man/30m2 is the most productive but cant find a document that references this. Could anybody point me in the right direction.....?

The project is a large industrial facility and we are installing Building services.

Any help would be appreciated.

Regards 

Mark