Thanks everyone, please keep your opinions coming! We live near (about 10km away from) a capital city in a hilly area. I've noticed that most people in our immediate area have raised carports because of the sloping sites. On one hand, having a garage could be a bit of a bonus, but it also shows that most people are happy with carports. Plus, we have the extra third parking spot, and there is still on street parking. We are not close enough to the sea for salt/rust to be a problem at all.

Hi there Nick, looking at your photos you look like a modern hamptons family which is lovely, do you live in a warm climate. I did like the photo you posted of the open plan living and think that you are on the right track with interior colours I have marked out how I would like the first floor, I have moved the kitchen to the end wall to line up with BBQ outdoor kitchen, plumbing for this would have easy access via the garage below. The white tiles look lovely and hard wearing for the entrance foyer and Rumpus room, the stairs in timber due to the amount of traffic even continuing in up stairs foyer kitchen dining living, Carpet in bedrooms and downstairs theater. Kitchen in white but possibly Shaker style white doors these are very popular. Using existing sliding exterior doors if possible from kitchen.Dining to verandah, I think this ceiling would be best kept white to help reflect light back indoors, the tiles appear to be in good order but you might consider changing these along with new upstairs flooring Some where for a laundry shute would be handy, actually a Dumb Waiter so you could move the laundry with ease both up and down! The roof painted a soft colour, grey/white or off white, the verandah railing could be the same colour or you could do glass. The builder had a very different style of house to what I think you are wanting. Your photo

first sketch out a rough idea of what you think you want, talk to local council and check on line building regs to find out what's legal, look around your suburb to see if there's any obvious sign of a similar conversion and knock on their doors, ask some builders for a guestimate of what they think they could do, draw up some more detailed plans and go back to council and the builders (aiming to have a firm idea of what's legal and possible before you start spending money on final plans) Keep in mind that it's sometimes cheaper and more efficient to build new rather than renovate so think about if it's a new room you need or that you want to renovate the garage roof space just 'cause it's available..perhaps there's a better spot elsewhere around the house or property..i suggest that you would need a building at least 8 m wide and 6 m tall to give a worthwhile attic room but dropping the ceiling height above the cars to minimu may help expand the available space but a floor plan showing ceiling and roof ridge heights could help invite more skilled suggestions than mine

Just off the top of my head, I would think that the addition of some insulation might combat a condensation issue. Also, will the room have heating?