Andy
My first advice is not to use a company to do the development. A corporate structure is undoubtably the worst possible structure you could use for any appreciating assets. I cannot stress this enough.
If you construct new residential premises then you must register for GST as you will be an enterprise
If however you build with the intention of keeping the house for rental then you will not be entitled to claim the input tax credits for the construction costs and if you rent it for more than 5 years the subsequent sale will not be subject to GST as it will no longer be regarded as new.
Margin scheme would operate if you were to build.
EG Property purchased $150,000
Spend $220,000 building (GST Incl)
Sold for $450,000 (GST inclusive)
For GST margin calcs $450,000 - 150,000 = $300,000 (not $450K - $350K)
Therefore GST payable will be $300K / 11 = $27,272 and not on the total proceeds
Hope this helps