you are not going to save 300k on retaining the existing structure.
if you are lucky and your house is in a area where there is good soil, you may escape with just doing footings or a raft foundation. this could save you maybe close to 100k from doing piling.
to do fencing instead of build brick walls as boundary wall will mean quite a bit of savings as well.
opt for those mid-range or lower ranged sanitary ware. a $300 toilet bowl compared to a $1k toilet bowl means $700 savings. multiply that by say 6 toilets will mean about $4k in savings.
don't splurge on smart switches, etc. do normal ones which are non-designer stuff
opt for half height tiling for toilets unless necessary like in shower areas. you save on the cost of tiles and labour for tiling. but still need to pay for plastering and painting which is still cheaper than tiling.
for flooring, marble would be most expensive. getting big homogeneous tiles for living room will look good as well. if you like hard wood floor for rooms, get parquet instead of engineered wood. else just do tiling as well for the rooms.
light fixtures can order online from China. can save quite a substantial amount as well though you may need to pay the electrician to install them since your main-con can't earn from this. for ceiling fans, no need to buy those $400 and up DC fans when a $200 AC fan will perform the same job. unless you want to splurge $3k on a 84 inch Haiku fan to make your house look grand, else getting a few normal ceiling fans or standing fans will also work.
the ducting for ducted aircon will cost much more than the aircon unit itself. get ceiling cassette units to save on the cost.
if you can live with exposed beams on the ceilings, tell your main-con or PE that you do not want to have false ceilings and ask the PE to design flat beams instead. a normal beam will be about 500mm to 600mm thick whereas a flat beam will be about 300mm to 400mm thick. no false ceilings = $$$ saved
for the roof structure, can ask the PE to design using steel columns and beams instead of RC. then do a metal roof. this will cut the construction time as well as lower the cost a bit.
sliding windows are cheaper than casement windows. no need to pay extra for low emission glass as normal glass will work. if want to block out the sun, do solar films later on.
doing normal metal railings will be much cheaper than glass railings.
anyway, there's many things that can be done to cut costs. how much the house costs eventually really depends on how much you want to splurge on the finishes. by cutting down on the luxury finishes, you can save quite a bit of costs on the construction.