Note:

Several of the included house plans include garages.  When a house plan includes a garage, the garage is included in the overall square footage given on the same page as the plan.  Since Habitat doesn't build garages, but the roof has already been designed for the space, the garage space could be converted to a family room or similar.

On a similar note, several house plans include second levels.  In all cases, the second level is to be neglected.  Space taken by the stairwell to the lower level would be reclaimed for other purposes.

Finally, several houses have complicated roofing systems and recessed walls, etc.  In all of these cases, the house plans would be simplified to the standards of Habitat.