Wouldn't it be easier to just box a 100 mm thick bench and use normal conc mix and reo? I am sure it would be stronger too? Just a dumb chippy here though.

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along under the window for towels for the vanity and opposite the toilet for the shower (extra reinforcing in the wall around the cavity slider) will make the wall wide and strong enough for a double height towel rail..and a half wall beside the toilet would make a better place for a toilet roll holder...nb the new wiw is based on assumed dimensions as measurements on the plans are not clear or always accurate so you should mark out the new walls to see if you would feel comfortable with the changes making sure that the small hall inside the bedroom entry doors is at least 90cm deep

I was thinking of options , and basically Kates sketch would be along the lines of what I am thinking . Just some obvious things -- you have a chimney -- do you want that in your Master ? You want doors off the front Den -- no matter what you do , that will cost reasonable $$$ , as you will have to alter the framing , maybe electrical , as well as obviously cutting through bricks . Could 'just' having screens and maybe even a small roof creating a courtyard in front , using the existing front door and probably creating a better entry experience if done right , be better than that ? Same with things like having an external door from a laundry/mud room ? If that was important , personally I'd incorporate that as part of the new addition at the rear -- its way easier to design framing and walls with a door included , as opposed to modifying a 60 year old building . Basically , we could question every room , and that is what the designer should have done . Without asking , I'd come up with a totally different design , but it would be what I would do , and maybe wouldn't be what you want . And as far as adding upstairs , again personally I wouldn't . Engineering wise its difficult , matching styles is difficult , and cost will be quite a bit more per metre than adding on the rear , but you may feel the advantages outweigh the costs -- only you know .