Marina is correct and we can't provide you with exact details without your providing additional information and in particular the approximate value of your current home and the amount owing on your mortgage. Then, one would also need to know the approximate cost of the lot and approximate construction costs. At least this would be a starting point.
For example, if your existing home was worth $500,000. and you owed say 50,000. is quite different than if you owed $300,000. on your current home. Also, what would you guess the lot and construction costs would be? I would suggest the ABSOLUTE maximum you would ever hope to be able to borrow from a conventional lender on a conventional basis would be 75% of the value of your existing home (less existing mortgage) and 75% of the value of your new home. In saying this, 60 or 65% would probably be more realistic. One mortgage for the cost of the lot and construction would be registered against both properties. This is what an interalia mortgage is and refer to my previous post. Also, there is the matter of debt servicing and do you have sufficient income to carry the construction mortgage and your existing mortgage?. As mentioned by leblanc, a mortgage is simply a document registered in the land titles office securing a "loan". You would have one loan for the construction secured by a mortgage over the new house (it would be a 1st charge on the property) and the security (the mortgage) would also be registered (interalia) against your current home. It would be a 2nd charge against your current home as your current mortgage is the 1st charge. Things can change depending on whether you are going to act as the general contractor, have a fixed price contract, or have hired your contractor on a cost plus basis. Be prepared to pay some fees like appraisals over both properties, legal, interest, bank fees, quantity surveyor fees (if required), etc. If you are buying the lot from a developer sometimes they have arrangements for "builder financing" and then you can buy the property outright when completed. An owner acting as their own contractor is generally frowned upon but again, this would be deependent on the equity in your current home and on the one you are building. Good luck.