There are certain "standardised" specifications and construction details that if executed by a volume builder can be achieved extremely cost effectively. Once you stray from these "rules" however, with things such as higher ceilings, skillion roof forms, high level windows or whatever, it immediately enters "custom build" territory, that will of course equate to higher cost. The "reasonableness" of this however needs to be rationalised from project to project, from builder to builder. Additional $90/m2 for skillion roof stick framed vs standardised truss construction is probably actually quite reasonable, if it involved reasonable spans, and it takes far more time/labour to properly pitch a roof from scratch (with a raked internal ceiling) in comparison to just throwing up pre-fabricated trusses, that involve barely any tools other than a nail gun. I'd suggest as a general comment, rather than rule out options at such an early stage, it's better to develop a vision and see how close you can get to it, get the design/plan sorted first, and then see where it's sitting and what opportunities there are to have raised ceilings or whatever it is and then methodically and logically work through prioritised spec adjustments if budget is becoming pushed with the scale/quality balance of the design. Be careful not to just make big design decisions because someone tells you it's cheaper or better or not or whatever one way or the other. Yes budget is always important, but don't allow fear of blow-outs potentially hamstringing/compromising the design process/journey and automatically ruling out opportunity. With the right designer you may come to find that with more efficient planning you actually can afford some of the interesting/custom features that appeal as well as all the function you require for your family.

The idea to extend out under the eaves was to add and use available space under the existing roof. you will see the 3 eave extensions marked with circles Both bedrooms would continue the bump out wall next to the robes for a desk space or other by using the bricks from the exterior wall and the return/Nib wall. The Nib wall between both the robe and desk would be framed out with timber and Gyprock so that all the original bricks ca be used in the new wall. A concrete base of approx 600 deep will need to be added to both new desk spaces. The third bump out wall suggested was in the laundry space which would reuse the existing sliding door and bricks and bring it inline with the kitchen wall, once again it will need a fill in concrete slab of approx 600 deep but no extra roof expense applies. The Carport I think has no walls so nothing to demolish there it would however need 3 new walls of brick or cladding as previously mentioned plus the extra narrow concrete slab on one short wall and part of the long wall. Stepping from the old now wider laundry into the new Master you would need a new small slab across the end of the old Carport. This section would need a small section of roof to either sit up to or under the corner eaves, this could be slopping glass or Metal. All renovations/ extensions will involve a need to match exterior walls which can be done by rendering or cladding, in particular the new walls ( the bedroom extension walls would include the existing robe walls) All of this only requires the addition of a small roof section in the corner of the laundry and the carport Eaves, a small sloping glass roof here would add some Wow factor plus the aspect is perfect.. A few small additions can make such a difference for a smaller outlay. A new double carport can be factored in to be done in the next stage as you have plenty of space to work with You mention the use of a Modular system, along with OKL and my suggestions you could have a livable shed costed by local Professionals. would be interested to see what the difference is

We have used a great builder that we know well and designed our current knockdown/ rebuild home ourselves with his input. The builder uses a draughtsman to draw up the necessary structural plans and deal with council /NSW planning portal requirements. We have done this twice before for a large extension and a partial rebuild on other houses. It relies on a great relationship with a a good competent builder but we have been very happy and certainly saved money on an architect .

theatre next to alfresco would block kitchen or family room from outdoors and you need wide views of the backyard from busy area if you want a pool but theatre can be swopped with master and space next to the kitchen was always a scullery and the computer nook is in the family room these rooms are reasonable sizes and furniture helps explain available spaces and i've shown the simple hip roof but keep in mind that this plan is more than 100sqm smaller than your original so would be much cheaper to build but you will still need external shutters to screen some windows for some parts of the day during hot weather