ok, here's my go: keep kitchen in current location. fill in wall between living and dining room. remove wall between kitchen and living, replace with steel beams and columns. the living room becomes the new dining room, with access straight from the kitchen. Possible skylight above the new dining room. Remove doors from kitchen or replace with sliders (if cavity available). The back bedroom can become a study. Now for the expensive bit: new building on the back (choose a style, keep the calif bungalow or go modern), all insulated double glazed etc..new living area, 2 beds, maybe an ensuite if necessary. the front bedroom could become the master with it's ready made ensuite. Link the new and old with a light filled living area. An architect will do wonders here, and will probably save u $$ on a poorly designed/constructed build anyway. I wouldn't go up with this house, sensible extensions and renos only.

i have a problem with the awkward location of stairs that wastes the sunroom, makes the rumpus room difficult to furnish and the front door has a view of the toilet ...beginning with central stairs and potentially modifying doorways to allow for the stairs without having to walk across the rumpus or upstairs living room attempts to avoid the big wide hallways affect without useful locations for furniture ..the rumpus room now has space for comfortable furnishing and private access to a full bathroom and good sized laundry...using the old sunroom for a compartmented bathroom retains the original walls and windows and includes space for a bathtub and shower with big views.. we should always include real scale sized furniture to establish the best walking paths across rooms and my suggestions also retain the size of the original kitchen to keep more of the original walls and ceilings and retains the proportion of the bay window and allows space for an alternate access to bed 3 (preferable the linen cupboard would remain in this location) and consider avoiding the enclosed pantry (would the door ever be closed?) in favour of a more open space for better light and ventilation and maybe a herb garden window box in the bay window with external blinds to exclude western sun.. there's a fridge closer to the outdoors, stacked wo and MW to define the pantry and only a small sink in the island for more useful set down space for shopping and serving meals

From a "passive solar design" point of view, everything is right. Rectangular home facing north/south. With north facing living rooms, rarely used rooms to the west. But one other thing to consider, plumbing. Locating it close together will lower building costs, & the wastage of water, waiting for hot water. So I'd consider swapping bedroom 4 for the western wet areas. Behind the kitchen is have the laundry, then bathroom, then linen, the bedroom furthest to the west. I'm not mad about western bedrooms, especially in hot climates. But in this case it sounds like it would be used sparingly. Plus if make sure there was no western window, just a northern one. Insulating the western wall well, & using a radiant barrier (reflective insulation). The approach will also significantly shorten the length of the western hallway. Greatly increasing the size of bedroom 4. Talking room sizes, & room numbers, from a sustainability point of view, I think the home is too large for what is likely to be a two person home for much of the time. The embodied energy (emissions created) from a new build home is huge, averaging 15 years of operational energy. I'd be looking to reduce the house size, by using multipurpose rooms. The occasional kids housed in the study, or the media room. This can work out great with clever design (like incorporating murphy beds). How will the media room be used? Will noise be an issue. Will you be happy to have it so close to the master bed? The building orientation/shape will be great for solar PV, with a large roof area facing north. I'd I was building nowadays, is be aiming for an all-electric home. Ditching gas - of its even available where you are - in favour of efficient electric appliances. Reverse cycle AC for heating (if it's required), supplemented by fans for cooling, hot water heat pump (extremely efficient), induction cooktop (sensitive, fast acting, easy to clean). All powered by solar PV. If it's an option, I'd look at 3 phase power, which will let you install a larger PV system. It will also allow you install a fast charger for EV, likely to be your next car purchase, or soon after.

moving the stair doesnt help the existing upstairs if your plan is to not change much. the living area would reduce from 3.5 to 2.5m width and would limit options unless you want to use this space as office/playroom. the existing door would have to be relocated to bed 1 and the green line is an existing beam which looks to be tight on the 12th stair down which is usually the minimum head height clearance.