We are in the same situation as yourself. We ended up doing 2.5m basement, 3m ground floor and 2.7m top floor with .3m in between the floors as we also had height restrictions on what we could build. We are also building with slope to our land. Our basement has garages, games room and a bedroom in it and we felt that 2.5 worked. Hope this helps.

You can have what ever ceiling height you can afford. But also think of what height windows, internal and external doors will be. What heating and cooling system you will install. What window treatments will be required for extra height windows, etc etc. Our double storey home has 2700mm ceilings throughout with 2400 doors and windows. This height is ideal. A gable ceiling in the living and dining areas, I think, has more character than a 5 metre ceiling in one room. Have 2700 ceilings throughout the remainder of home. Depends on the aesthetic you are going for, to what the design could be.

Is there a particular reason you want to lower the alfresco height ? If I had to make a choice , I'd almost prefer to go for 2.7 indoors , 3 metre alfresco , because thats where you want that open feeling , the luxury of space and height , moreso in in an alfresco . As far as being noticed , the only time it wouldn't be too noticable ( IMO ) is if the alfresco floor was lower than the house level -- if you step 'outside' through a standard or sliding door , and the roof feels lower , I suspect you would notice it . That , afterall , is why you go for a 3 metre inside height , to feel open and spacious .

Agree with Siriuskey above. Hard to decide without seeing the style and layout of the house. If the expense of 3m windows is prohibitive, stick with the 2.7m ones and go for the 3m ceilings in the larger spaces. Install curtains at ceiling height so when drawn, it has the perception of full height windows and gives you nice simple lines. See below. Smaller spaces like powder rooms and studies perhaps you could have lower ceilings but that doesn't always save any money but small tall spaces need to be treated differently with the decor. Also heating in winter in Melbourne will have an impact on your energy bills. Heat rises so having low down heat like in slab heating will make the space more comfortable for the dollar spent. Think about fans in winter to bring the heat back down as well as for cooling in summer. Are all the downstairs spaces flat ceilings? I.e, no double height or sloping ceiling spaces? The amount of light in a space doesn't only depend on how high the windows are. Sun shines downward and so having windows go down to the floor and spread across floors may have more impact on light levels. This depends on where you place furnishings, what floor coverings you have etc. In bedrooms for instance where there may not be great views and you want to maintain privacy, I often prefer to have narrow tall windows carefully placed rather than the typical developer wide high sill windows centrally located on the outside wall. Are you on a suburban or urban site of in the country? That will impact privacy vs views. How much overhang or screening will you have to prevent summer overheating? If someone is designing this house for you, they should deal with all these issues with you. My feeling is there is much more complexity to consider than just your one query. Share your plan and elevations for us to help you more, even if they are just sketches. Good luck, C.