As mentioned above, if you are able, there will be an assignment clause in your contract, in which case you will end up paying capital gains tax or investment income tax on the money you've made (depending on your other income operations). If you're not sure what that means, ask your builder, or stop into my office for 5 minutes and I can take a look for you.
BUT as was said, the likely scenario is that you CANNOT assign it, in which case you will have to close on it, and now pay all the closing costs and land transfer taxes as well. Also as mentioned, there may be HST implications. THEN consider an immediate sale might result in paying capital gains taxes.
IF you move in as your primary residence for a while, THEN sell it (time is a grey area, so if anybody tells you 1 months or 3 months or 1 year don't believe them - it has to do with patterns, red flags, and mainly you naming your principal residence for that tax year), then at least you avoid the HST implications and capital gains tax - if it truly is your principal residence. Don't mess with the CRA
Good luck with it!
The one common answer - I agree, talk to a professional - and I'm talking about a TAX professional... Don't talk to somebody that just tells you what you want to hear
Make sure you get the exact details you need, so you understand the implications of whatever decision you make.Good luck, let us know how it goes!