It sounds as though you purchased and took title to the lot seperately and prior to the house being built. Normally the builder has the lot until the house is complete and house and lot title transfer together. The builder is responsible for paying the year's taxes, and there is typically an adjustment made by the lawyers at closing to charge you a prorated amount for taxes for the period of the year you actually own it.

If you bought the lot on your own, seperately, then you owe taxes on the lot from the point of possession regardless of whether the house was built.