If a new home is constructed on a piece of property already owned by the borrower is the HMDA reportable loan considered Home Purchase or Home Improvement?

It would be a purchase.

Agreed. If no dwelling currently exists on the property then there can be no improvements made to a dwelling or the property on which a dwelling is located. Therefore it would not meet the definition of a home improvement loan.

Constructing a home is purchasing the home. You may be purchasing it one section at a time but you are still purchasing it. The loan (const/perm) is secured by the dwelling being constructed therefore it meets the definition of a home purchase.