Advise on duplex build in Melbourne

Hey all, this my first post on this forum. Reading through the posts have been very informative.

I was after some advise on a builder project I plan to do in a south east suburb of Melbourne. The 700 sqm approx corner block has a 4 bedroom old house on it. The property is owned outright, so there will be no land costs. The plan is to knock the old home down, and either build two large two storey duplex homes, or build two seperate freestanding two storey houses.

I guess I am just after some advise on which option would be better for $ return. As I assume the duplex would be cheaper to build, but I assume a freeholding home would sell at a higher price?

The plan is to sell one property, and live in the other.

I have budgeted approx $800,000 for the total build. The finish would have to be of high quality to match others in the area. Quality duplexes are currently selling in the area 1.1 – 1.2 mil each.

Any advise from those with previous experience would be great.

Thanks in advance.