This looks pretty good, there is no other windows or access to bathroom nearby, I’ll be replacing garage door with windows to match the others at front of house.

The bedroom only needs to fit a Aust standard double bed with double wardrobe and potentially a desk.

these rooms are good sizes for their purpose and the small study allows for other family members to use the space and also overcomes the challenge of the location of the door into the house but we need to use metric measurements for Australian plans and old garages often don't have the waterproofing under the floor so consider suitable lining for walls and floor to avoid dampness and if the new windows are made to suit the inside spaces and not the size of the old garage door..smaller windows will look better from outside and less like a garage conversion and the rest of the gap can be filled in with suitable framing, bricks or cladding to match the house

I would contact a builder or designer first... before you do anything.... do you live in Queensland?

If you want your build to be insured against rising damp I would recommend getting a licensed builder to do this.

I'd contact a designer first and have them inspect the proposed. They'll check to see if its achievable within budget as well as building codes.

I've done quite a few and they have a few hoops to jump through to be compliant. Depending on council location of car parking needs to be considered so a new area may need to be required for example.

Hi, I have already spoken to Council and it is all ok, have spoken to prospective builder and all I need to do is waterproof and raise floor a little, just trying to maximize space. Thanks everyone for your advice.

Could you upload a image of existing floor plan, just for clarity.

Certainly can, this is a photo with drawings already for the split but at least you’ll get idea.
Is 2.7 x 3.9 ok for a bedroom?

Another plan from original drawings

I would make the lounge as wide as possible.

Could also put in a kitchenette. Area for arts craft clean up.

Thanks for the ideas. Definitely don’t need a kitchenette as this bedroom is for my daughter who is a tween and lounge is just to let the kids have their own area (and to make noise).