Is a home under construction a building or a habitation?

A habitation is a structure "adapted for the overnight accomodation of persons." I would suppose it is up to the jury as to whether your building is near enough completion to be considered sufficiently "adapted" to have become a habitation.

From Matter of E.P., 963 S.W.2d 191, 193 (Tex.App.--Austin 1998):
The controlling standard for determining whether a structure qualifies as a habitation under this definition is expressed in Blankenship v. State, 780 S.W.2d 198 (Tex.Crim.App.1989) (opinion on reh'g). The Blankenship court noted that determining whether a structure is or is not suitable for the overnight accommodation of persons is a "complex, subjective factual question fit for a jury's determination." Id. at 209. Factors to be considered include whether the structure was being used as a residence at the time of the trespass; whether the structure "contained bedding, furniture, utilities, or other belongings common to a residential structure"; and whether the structure was of such character that it was likely intended to accommodate persons overnight. Id. The Blankenship court...noted that "[a]ll of these factors are relevant; none are essential or necessarily dispositive." Id.