As a ballpark starting figure use around $1800 - 2000 per square metre. This just a ballpark figure to use as as starting point and can easily vary either side. So for $150k for that size dwelling is only around $600 per square metre...... Contact a local designer (the good ones understand budgets and costings) and start from there. Material and product choices will vary the cost to construct greatly so keep that in mind. Building costs are always increasing not going down.

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 .

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.

The new door and frame and window frame, including glass, to purchase would be approx $1400. Is there $5000 worth of labour to remove the existing and prepare the walls for the new frames?? …….I would suggest to get more quotes.