Subdivision costs

HI All,

I'm new to the forum…and looking for some advice.

I have recently bought a 1900sqm block with an exsisting house on it. The land has got a subdivision approval for 3 lots (which was approved in 04 and is valid till next year). Some of the conditions relating to subdivion are:
1. Creating an easement for stormwater
2. Exsisting trees – and retaining couple of large trees
3. Putting a driveway
the block is narrow and long (20×95), There is approval to sell the land on its own or build duplex on each lot, Each duplex will however be on ONE title.
My questions are:

Q1. Can anyone please advice on the costs for putting an easement for storm water drainage – it will be around 90mt long. the block backs onto a council reserve.

Q2. is it better to sell the land alone – after demoliting exsisting house and complying with subdivision conditions. OR should i build duplexes – Will the land price increase with the FHOG? 

Q3. Is there a way i can sell each duplex seperately, even though the council has adviced that it will be on 1 title – can i strata the duplex.

btw..the site is in a established area, west Sydney. there is not much land available, its mostly townhouses around it and its close to the station and shops.

Thanks for any comments/advice

Jaidee