You will need a planning permit. Did u get planning permit to convert garage? If not that could create issues. Have you talked to council or a local planner. Can you share your floor plans and rear elevation.

Hi, I don’t have any floor plans yet, just an idea and sussing out if anyone thinks of something better or cheaper.

This would still be less than 60% of built space on the land so I think it’s feasible.

We used a company to convert the garage who submitted all the appropriate planning requests etc. that was about 3 years ago now.

Also the elevated backyard is 600mm off the courtyard/house level. Any suggestions much appreciated!

Do you have existing house plans

whole house plans will make it easier for us to understand the existing house before we can make any suggestions.. try adding walls lengths to each room in your sketch (make a new sketch without all the extra writing) what is the distance from back of house to change in backyard levels?...does the backyard slope or are there steps connecting two flat areas of backyard and if so how many steps? what is the difference in height between the ground floor and the highest backyard height and how do the upstairs rooms relate to the different levels?...a Google earth view and photos would help

Being able to open the extension on both sides means that you’ll need to build bigger than you functionality need to allow for walking space. Given the age of the existing house and the different yard height a living room that is attached by a link and set to one side may work. But the plan people on here need more info.

If you just want an entertainment room your kids are old enough for it to be a separate structure in the backyard. Move it back as far as possible. Avoiding touching current house will keep costs down. Allow for plumbing to be added for a small bathroom

An old plan will do as we can see the new kitchen and laundry via your previous posts