You can treat a home under construction as a qualified home for a period of up to 24 months, but only if it becomes your qualified home at the time it is ready for occupancy. The 24-month period can start anytime on or after the date that construction begins.
If you are building a home on a construction contract or you have a loan on the land, you can mark that you own a home and deduct the interest and taxes that you are paying on the loan or land. If the home you are building is an end loan (meaning you pay only after it is ready to move in), then you will not have interest or taxes to deduct as the builder or bank is paying until you close.