Lot size and Floor space ratio question

It's not the size of the house in M² it's the actual footprint of the house. So your 375m² design may be 225m² downstairs and 150m² up stairs.
The plot ratio is based on the 225m² and not the combined total.
The fact that your architect doesn't know this is a bit of a worry.

Hi Dee
I'm in WA but from what I have seen the whole plot ratio calculation is similar everywhere, but I'd encourage you to have a chat to someone in your local area that is across the planning requirements.
There are a few things that need to be considered when designing a house. Setbacks from the front and the other boundaries (including openings), over looking of second storey windows and finally the plot ratio.
In WA our zoning is expressed as open space, so the percentage of the block that is still open or unbuilt after construction. Different zoning types have different amounts required.
The calculation uses the entire footprint of the house not including eaves, so basically external walls. In WA if your alfresco is open on 3 sides you can get a provision back for a certain amount but not all of it and porticos are similar I think.
Garages and voids are all included with no provision for them.
You absolutely need to get help from someone who understands the planning requirements for your area. Having said that you sound like you are only sitting about 40% coverage as described so I'm sure you are good.

Depends on your residential zone and Blacktown's allowable GFA. If your architect doesn't know, I would also be worried as that's the point of an architect to design based on the requirement of the council and you.

I would call the duty planner at Blacktown council and ask.