with so much available space adding an upstairs would be a very expensive choice and, as previously advised, you need accurate dimensions of the house and all the setbacks along the whole boundary and a local building designer can confirm measurements and make specific suggestions but something like this might suit

Thank you @oklouise - propose additional floor size upstairs will be around 75m2, in which accomodate 1 master with WIR and ensuite, 1 x bath and 2 x bedrooms. I've spoken to council, and from the observation, the room to be available for us to extend horizontally without violating the rear setback rule (red line below) is very very minimal. (We are in The Hills Shire Council)

Hence ironically we are seeking to redesign the downstairs based on the floor size we are having ( approx 125m2)

i've used the 4 metre diagonal setback so some of the dimensions shown must we wrong so again i suggest that you employ a local designer who can make on site suggestions

What to you require downstairs. Keep one bedroom. Definitely place for stairs? A lounge? A study? A dining area? An extra lounge? I would not reduce the size of the bathroom or toilet. It needs to be a family home space. You could put washing machine into bathroom if you want to combine kitchen and laundry into one room.

@dreamer yes keep one bedroom/study whatever we like to call it. Definitely, spaces for stairs is required. I am looking to have a lounge, a living, a bathroom, of course a kitchen. I was thinking to locate the stair next to entry (bedroom 3 area). Laundry is the least after. I have been witnessed some new apartments those laundries are just sit within cabinet. I am happy with this setup BUT not much idea where to put would be the most effective.

@oklouise thank you for suggestion - yes we are in talk with various builder to start kick the ball rolling...

Regarding the laundry. If you are having a bath upstairs in the bathroom, then take out the bath in downstairs bathroom and put the washing machine and sink in that space. Personally I would keep the bathroom and laundry as is. It’s a family home, and both rooms will be used every day. You have the room, since bedrooms are going upstairs, to keep the laundry and bathroom as is. Why compromise with a washing machine in a cupboard?

Thank you @dreamer - my intention is to create more spaces as possible for the kitchen and living room. I found I might require more space for the kitchen, than a laundry. My old place has got a bath setup but I never ever use it. It is preference (a personal one) I would say :)

Something like this. Being aware of structural walls that will be needed for upper storey.

Bed 4 is small bedroom. How about master on ground floor and second living upstairs.

master on ground floor could be great but the kitchen is still very small

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@oklouise@Kate much appreciated for your input. Our perference is to have master located on upstairs. BUT I must have to say I very much like the idea @oklouise proposed some hours ago about relocating the laundry and garage door location make living and kitchen more spacious. I hope you do not mind I will take this idea and share among our prospect builders.

just for comparison and using Kate's suggestion for a master downstairs and extra living upstairs could be like this

Another idea is to have the stairs in the open plan family keeping that room as a media /office, I also like the master downstairs with 3 bedrooms and a rumpus study upstairs for the kids and future adults

@siriuskey thank you so much for giving us another alternative. It looks refreshing. My Mrs and myself still have a strong biase toward having master to locate on the upstairs. Definitely, I will take this to our builders and obtain a second option.

You could still have your master upstairs and use the downstairs for guest/Parents,