If we don't completely pay off our current mortgage by March 2019, will that slow down the construction timeline?

The more we can pay down our current mortgage, the better our financial position to begin construction will be. If there is a balance remaining on our current mortgage when we are ready to break ground, we will likely use part of the construction loan to make the final payment. We would need to make sure in that case that our combined gifts and the construction loan would complete the building.