I think I saw the house you're interested listed on one of the property portals. if i got the correct property that you are eyeing on, your diagram is a bit wrong as well on the plot size as the kink area is actually a sharp corner and that area is almost negligible. the property I found is not in a normal landed house residential zone but is in a 1.4 plot ratio zone. this means that the landed houses in that zone can be built up to 3.5 storeys high which is why you see some low rise apartments there.
if you want to do a recon and add another 1.5 storey, you may run into issues with the foundation especially on the existing soil conditions. since there is a possibility to build up to 3.5 storeys, it would be better to just do a complete rebuilt which can give you maybe 5000 sqf built up area rather than try to do a reconstruction and retain the existing setback. also (if I got the correct property), the 2nd storey at the front is cantilevered with no columns supporting it at the bottom. if you are to build a new storey or attic above the 2nd storey for a reconstruction, you may not get the approval from URA since you may need to add new columns to support the new storey/attic within the setback area which is not allowed.