It should be understood, the value associated with these 'unconditioned spaces' is calculated differently. A lanai, for instance is open space that lies beneath the main roof structure of the house, typically the area adjacent to the pool. Since this unconditioned space is beneath the roof (not an add-on flat roof) the value is significantly higher. These spaces can be significant. The same would apply for an attached garage sharing the same roof line as the house.
It is not uncommon to see over 1,000 SF difference between total square feet and conditioned square feet. This space can not be discounted as having no value.
A screened porch is most likely INCLUDED in the total square feet. You should see it itemized on the tax assessor's report. In Willmington DE, lanais may not be a factor, but garages and porches, as you identified, will be evident.