Hi Josie love that area! I used to live and work nearby..... Without knowing the budget or what you intend eg living there / going to make it a family home / investment property, there are quite a few options that depend on answers to the above. sticking with your original question, there are companies around that prefab the entire second storey that is installed in 1-2 days ( the structure install only- doesn't include roof prep etc) This includes the roof, finished walls (exterior and Gyprock internals). Your builder then needs to do the staircase, paint, finishes, bathroom fit off etc but it is very cost effective and is in the $1500m2 range. you obviously need to renovate the existing but $2500m2 is a good guide as others have said.... It really depends on your taste and again, whether it is a home for you or an investment. I understand the detonate thinking, however the existing layout can be worked with. Before detonating, it would be smart to speak to the Council about your setbacks etc of the existing home as quite often the existing home is closer to the boundary than what a new home could be. For a quick and dirty answer to replacement cost for a new double storey home, speaking to a double storey project builder with standard designs will allow you to see what works without a lot of expense as they can advise if their designs will fit. This will give you a budget and allow you to see some designs. you may even see some ideas you like and with which a designer can build on (no pun intended ) cheers

It depends on many factors, some interrelated, some not. Unless you are dealing with more off the shelf/volume/standardised design solutions I'd be somewhat wary of utilising any fixed overall /m2 calculations to base your decisions on what you can or can't achieve or should/shouldn't do for your project :) We prefer to present/offer our professional opinions of likely construction cost by collating the projected likely trade breakdowns carefully considered for each particular scenario proposed, factoring in various aspects such as quality, site, scale etc that will ultimately contribute to influencing the number involved. Have found this to be a more accurate/realistic approach than simply applying $$/m2

with 22m to the rear boundary my suggestion would be to build a separate dwelling using a variation of the house plans on the yourhome.gov.au website and connect the two building with an extension of the existing covered deck to make a shared alfresco...the new building would have a raked ceiling with the roof sloping down from north to south.. the web site has detailed plans and specs suitable for Adelaide that a builder could use to estimate costs eg 150sqm @ say $2000-$3000 per sqm depending on materials and quality of finish and contingencies but you'll need to discuss your ideas with council as there's also a restriction on the total area of the block that can be covered with buildings but something like this could work with the mancave extended from the carport but you also need to ensure enough light and view into the family dining and kitchen

unable to read all the dimensions but assuming a narrow block of land with zero set back on the garage you would lose extra width upstairs with the additional setback needed for upstairs, none of your examples could fit the available space and there would be a lot of wastage upstairs and down with hallways and stairs and, if a suitable plan could be achieved, there would also be substantial costs ($10,000?) in council, design and plan drawing fees plus costs of living elsewhere for months to build a second story and an upstairs would not improve modest downstairs living and outdoor areas...my suggestions would be to make improvements without over capitalising and could be achieved in stages eg new laundry and move meals storage, (nb this allows the old laundry to continue to function pending completion of new laundry) rearrange bedrooms and replace old external laundry door with window and cladding, enclose the alfresco with insulated wall and cladding), move sliding glass doors and use windows to suit direction of sunlight and outlook in new family room and add a tiny handbasin (inside the wall) to the toilet and, although it's not legal to use a garage for living areas, consider a partition to create a storage area (keeping the garage door) and make a rumpus room with skylight in the garage and leave cars outdoors...these changes creates 4 bedrooms, bigger study, new laundry with storage and a much shorter inside walk to outdoor drying area, generous dining and family room and you could add paving outdoors with a giant market umbrella to replace the alfresco ...unfortunately you will still be astounded at the cost of any changes but hopefully a staged approach would make these changes manageable and you could save more with DIY freestanding flatpac wardrobes, family room and study furniture and your carpenter/ builder might be happy to credit some costs for help with demolition under supervision