Proposed layout

With input from a couple of the architects I interviewed, including Nicole from Bespoke Architects (with whom we went), we have finalised a layout.

We went through several variations, and options, but for cost as well as functionality reasons settled on the plan below.

Downstairs
Unlike my original plan, the laundry stays where is it, and there is no change to the footprint (↘$). Instead, the kitchen and lounge area swap. This puts the kitchen in the north-east corner of the house. The stair gets moved from it's current location to where the pantry/crockery cupboards are, going from near the back door up towards the front of the house. The telly and some bookshelves go under the stairs. The laundry stays put, although the door gets moved a couple of metres. The access from front to back of the house is restored to where it almost certainly was in the original house, which is where the toilet/stair sit presently. The current bathroom will become accessible only from the master bedroom, and the present office/junk room will become the general access bathroom.

Pros

• Kitchen in corner, so rangehood vents outside, windows near space, pass-thru to barbeque on back deck;
• Full general purpose bathroom, including full length-and-depth clawfoot bath
• Lounge area out of the way, less likely to be interrupted
• No change in footprint, keeping costs down; no need to relocate washing line (Hills Hoist rotary)
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Cons

• General bathroom is very close to living areas, resulting in possible performance anxiety
• Passage from front to back of house is pretty narrow (900mm), being the original width
• Kitchen (or more specifically, the pantry) isn't as humungous as I'd hoped, but it will be perfectly adequate

Upstairs

Due to the insufficient headroom of the current layout, and having reviewed a number of possibilities, the only option is to pop the top. To make it look less silly/peculiar, Nicole has suggested doing it somewhat off-centre which works okay. There will also be a large dog's house (gable) dormer roughly centred on the east roof, allowing enough room for a third (guest) bedroom upstairs. There's a smaller gable dormer to the west, bringing light to the stair and upstairs passage. The main ridgeline will be extended to bring the laundry under the main roof (rather than its current skillion roof). Whether we stick with tile, or switch to corrugated iron will depend on the relative costs. 

Pros

• Get an extra bedroom at not-too-excessive cost
• Bedrooms plus bathroom will accommodate the soon-will-be-giant boys
• Boys' bedrooms will have two operable windows, allowing air movement without having to have their bedroom doors open (noise issues mainly)

Cons

• The off-centre pop-top is the best of a not-great list of options. I don't love it, but I can live with it.
• The pop-top is inset somewhat from the main ridge (to stop it looking really weird), but this means the boys' bedrooms will have variable height ceilings. We'll need built-in robes or something into the lower-ceilinged space to avoid them sconning themselves
• The guest bedroom has only one window, which will probably be an opened-envelope shape, which will make it (a) un-openable and (b) hard to put a block-out curtain on. I think that will need to change to a rectangular window, casement if possible, awning if required (for overlook reasons)

  

  Gable dormer, with open-envelope shaped window