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

living in regional NSW i'm sorry to say that the cost for building "in the country" seems much more expensive than on the coast.... plans i've done for several friends on the coast have been about 20-30% cheaper to build than locally (transport costs. less competition and volume discounts?)...i suggest you check some of the building companies like Gardner, Beechwood, Rawson homes etc building in your area for a comparison price

We too are about to build though picked a project home builder because of the 'volume build' cost savings. But we are making modifications to better meet our needs. Our initial plan was heavily customised and came in around $150k over what we wanted, but that was highly spec'ed insulation with double glazed windows, cathedral ceiling, very large rooms, 2 bedrooms with ensuites plus another bathroom and toilet, plus another 2 bedrooms, large undercover alfresco, very complicated roof line (apparently that really adds to the cost)... so we learnt a lot of lessons from that. We have scaled back the size of rooms, gone for a simple shape/roofline and will add character through textures (timber panelled walls, etc), some of which can be added later. We have maintained top notch double glazed windows and high R factor insulation (Southern Highlands location, so you pay either way if you get that wrong.. up front or in excessive heating and cooling bills), but cut back to only 1 bifold door and embraced good old double and triple panel sliders - a huge cost saving in itself. Would also love a cathedral ceiling but may need to lower our expectations there. I have to agree that a price starting with a 3 at the front - or even a 4 - does not seem at all realistic for that house. If $360K is breaking the bank, you really need to think about how you can meet your needs in a different way. I'd start with design... would a 2 storey home offer cost savings? Less roof for the same floor price? What about attic bedrooms and/or an attic activity room? I don't think attic rooms are actually that cheap, because of the need to strengthen the floor/ceiling, but it would be good to ask. Do all the children need a separate bedroom? If there's a significant age difference between eldest and youngest, you may find you can get away with younger siblings sharing for a while longer; perhaps even till an older child heads off to Uni?? Think of all the materials you will be chosing and the impact those will have on your home. Number and type of windows, floor coverings, tiling etc. I don't feel your rooms are overly large, but agree the large Media and Activity rooms are expensive choices. Personally I'd keep one dual purpose room, mainly with the kids in mind and smaller than either of those. You really only need kids tv and computer games out of the family room/lounge, don't you? The scullery and parts of the Master suite are large and the bathroom between Bed 4 and Bed 5 is a waste. I'd put Bathroom 1 - with the only bath in the house - and the loo side by side and accessible from near the kitchen area. I've seen some homes with a 3rd toilet in a large laundry, which can be handy with a horde of kids around. On a different note, that laundry seems way too small for a family of 6. You want those teens doing their own washing and ironing ASAP... they'll need room to do that. And you don't want to skimp on windows, insulation and eaves that suit your climate - they can really help to reduce your home's running costs. Re the vergola suggestion, I'd steer clear of that. They are costly, not really water proof and don't age well in my experience. A few friends have installed them and all have shown 'buyers remorse' quite quickly. An under-roof alfresco is pretty nice to have, but even a solidly built pergola with a simple sun/rainproof roof is nice too. As for the cathedral ceiling, I LOVE it. Opens a house up beautifully. A half cathedral (that's not the right term but i hope you know what I mean) may reduce costs a little. Otherwise, use your windows to maximise light (but not heat), raise the ceiling heights throughout the house to 2.7 (it's usually fairly cost effective to do that) and ditch the cathedral ceiling. Good luck with it. It's an exciting project.

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