our neighbours did this last year. they extended up, put in a new kitchen downstairs. the upstairs included 2 bedrooms, the staircase and an ensuite size bathroom. it took 5 months (good going i reckon!) and cost all up $203,000. this was with a builder (not an architect) and the big money drain they reckon were the dodgy draughtsman drawings as the guy didnt take into account the specs on the stairs and when inspection came for the sign off, the council found that it wasnt compliant (by like a few mm!).

they had to move out, so shacked up with their mum and dad. but if you dont have that option it's worth budgeting in a rental.

i have been horrified by how much it costs to get building works done here. but also i think we have added costs inner city as the blocks are really tricky and there are all these restrictions on access and heritage etc etc.