The garage on the side of my detached house is about a meter further out than the line of the house. I want the extension over the garage to come to the front of the garage but the architect informs me that the extension needs to be 40cm behind the the line of the house, is this correct

 

Yeah it's pretty normal. 

 

You could apply to do something else but I'd say it's unlikely to be approved.

You'll see the word "subordinate" used frequently to describe this.

 

Sounds like your architect is just trying to set realistic expectations of what they expect would be approved locally. It would be rare to get permission to build over a garage forward of the main property line and examples I've seen haven't looked good.

It depends upon your LPA’s policies. If it already comes forward, you can still make the front element appear sub-ordinate/servient.