Okay that makes sense, so you don't think subdividing to a battle axe and leaving the existing house is using the full zoning potential? I as under the impression to stear clear of 2 story in this area as well though.
There are options options options. And of course depends on what conditions they put on R20/40.
You need to do the sums on all of them and work out what is best financially
#1: retain and build behind
#2: retain and sell rear block
#3: retain and build a villa at very rear of block then demolish front house and build 2 there.
#4: demolish and build 3 single storey could be 3 x 3 bed, 2bath with single garages
#5: demolish and build 3 villas of 3 x 2 with double gge , 2 x 2 single gge, 2 x 2 single gge
The sums should lead you. Surprisingly it is sometimes the simplest of retain and sell off rear block that creates the best % return.