Hi Boko,
I would firstly get a couple of real estate agents to give you an evaluation on both the land and the existing property as being subdivided (do you have plans and permits or are you guessing? ). I would also have them value the plans for the new property. If you are to obtain a loan and build, the banks will also do an evaluation on the property and its always a good thing to know what that evaluation is (keep in mind that it is a conservative estimate around 10% less). If you are to build, i would also get a builders quote for the new property. You have other options available to you, such as selling off the plan with a contract from a builder (providing you get the price you want) once again selling off plan normally results in a loss of about 5% – 10 %. Or if the figures dont stack up then you might want to sell the land as is. Don’t forget to include the costs involved for plans and permits !! You need to do the research and find your best option, i wouldn’t bother building for a profit of 10-20k if that is the case.