Just trying to study your floor plans which unfortunately I can't read so can only guess at the room use. I have no idea of the room sizes either, but I'm just wondering if you have actually done furniture layouts for each room to scale. This is really important. It seems to me like you have some great ideas you have picked up but then tried to make the rest of the house fit around them. How many people do you intend having live in the house? You have a lovely generous ensuite with a private toilet area for two people, but then have a main bathroom for how many people, jammed into a space almost half the size of the ensuite. Even the toilet is in there with no privacy whatsoever meaning that entire space can only be used by one person at a time. If you are planning this for kids, that will translate into lots of fights and arguments. If you want to create a really luxe and practical space, create a separate enclosed toilet, the shower and bath in an enclosed space and the vanity open for use at any time. This would be more in keeping with the standard you have set in the ensuite and balance the house so much better. These sort of details have a major impact on the overall value and attractiveness of your home. I don't know what you plan for the open space upstairs, but with a window along one wall a passageway the other side and only minimal wall space your use of this room will be severely restricted. Similarly the room under it will be very limited in its use. All the more reason to plan furniture to scale. I have seen master bedrooms done with the double doors which look very grand in the right sized room but in the wrong sized room, it just looks ridiculously pretentious to fling the doors open with a flourish, take two steps into the room and fall over the end of the bed. I'm sorry if I'm sounding harsh here, just trying to use my years of experience to help you create an amazing house. I've seen many pitfalls and I know how easy they are to make. You obviously intend spending a lot of money and you have some great ideas judging by your Ideas Books. Create more Ideas Books to cover every room of your house, then go talk to an architect, show him/her what you want, your Ideas Books will be invaluable to understand your likes and tastes and let him/her create something unique and special for you.

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

For those still reading this thread. We just recently completed a bathroom reno in almost identical sized space though turned the other way. A lot of reflection in the photos sorry but there is a lot of glass and mirror. A cavity slider, while an added big expense, made the whole thing possible. To create the space I used the adjoining separate toilet room for the 1200 x 900 shower and shallow tall cabinet on the left and gained a closet in the hall outside with the rest of that corner. I also recessed the towel rail which helped give room to the shower entrance and not stick out in front of the loo. Wall hung vanity and toilet. Porcelain wood planks on floor with underfloor heating and also used in shower base. Excuse the towels, they are not my normal ones ! It was a big decision about joining the rooms and whether to keep the bath / shower but I think it paid off. There is just the two of us and it's a real pleasure to use and no longer feels like "standing room only"

I was also going to suggest that you drive around your area looking at any new builds, who's building them and what Architect/Design firm has been involved. You could introduce yourself to any builders that you felt you might be interested in working with, call the Architect etc to discuss. If you can find locals to do the Job that's a plus, if there are any problems they are on the spot to sort. One of our Houzzers had a very bad experience where the Structural Engineer changed something without consulting anyone which ended up causing a major problem and expensive to boot. As annb has rightly pointed out you don't need to employ the Architect for the complete job as this will be more expensive, if your builder is good at his job he will manage everything with your input. Architects plans can be more detailed/complicated and sometimes there may be the need for the builder to check back with him/her. Big plus is that you are building where you live and are able to keep tabs on how things are going, very exciting time for you and your family