Here's a possible idea that won't need any walls altered -- I assume you know what a Butlers kitchen is ? It can essentially hide any prep and any clean-up . So like everyone else , I'm assuming that weird room ( the old laundry ) still has plumbing . So you could get half your kitchen installed out of sight in there -- a large pantry to the ceiling , at least a sink but maybe the dishwasher too , possibly even the fridge , cupboards for pots and pans , etc . Your main kitchen can then have a very pared down version of a kitchen . Obviously the oven and cooktops , probably a sink , maybe the dishwasher or a second dishwasher , a bit of bench and some storage and a bin . Basically , the showy bits . Lots of space ! And while you didn't ask , depending on how lives there and ages , I'd aim to adjust that back area ( Bed 4 and the laundry and 2nd WC ) . I'd possibly close off the door between Bed 3 and Bed 4 , but add one from Bed 4 to the WC , but then have a cupboard or even just in a row along one wall , with the washer and dryer in , but add a shower , then nice floor tiles -- basically an en suite / laundry / WC . Probable cost in the $10-15k region ? Just a thought .

You don’t need a builder to remove a shower. Are the shower/headtaps and tiles staying (hard to get a base out without damaging the tiles)? Who’s responsible for rubbish removal?

I would work out how many square metres you are adding and renovating. Then times that by $5000 per square metre. But there are so many variables. What is the house made of, brick or timber, what is the roof line to be able to be adaptable for an extension? Are the floors concrete or wood, to accommodate changes in plumbing? What specifications are you after, low, middle range, or fancy? Is the land flat, do trucks have good access…..?? Etc etc. only a few years ago I would have had said $3000 per square metre, but now…you need good firm quotes….and good firm time lines….so that is why I have said a very unsure $5000 per square metre.

First up talk to your local Council around their requirements. You may find there are hoops to jump through. Have you had a price from a plumber for the sewer connection? That will be the most expensive part. And for materials - there are generations of outdoor loos made out of corrugated iron - so Colorbond would be a solid choice.