Splitting a property off your principle place of residence is a reasonably common practice. However, in trying to determine where you stand tax-wise the waters can easily become a little murky.
You probably already know that your principle place of residence is exempt from capital gains tax. Unfortunately, once you subdivide, only one property can be exempt as your principle place of residence, so the other property (or properties) will not be exempt when sold. The cost base used when determining the Capital Gain on the non-exempt property, will be a portion of the original purchase price of the entire property. You will be eligible for the 50% Capital Gains Tax Discount, where you have purchased the property (not subdivided) more than 12 months before selling.