Habitable generally means functional and environmentally safe. 

We focused on building Energy Efficient & Sustainable Developments which reduced environmental impact, used high-quality durable materials, and included energy efficiency improvements. 

I have been in a couple of houses with later-add basements and the most common thing I've seen locally is a basement footprint that's about a foot or two smaller than the outer wall of the house all the way around, with a pony wall poured or blocked on the inside around the perimeter to provide stability to keep the walls from kicking out after excavation. 

@Patrick Q.I've used Erik Zerrenner from Premier Home and Environmental Inspections services and would highly recommend.

The reason is that there is uncertainty and cost involved in taking land and building on it, mainly permits, utilities, and potential soil/ environmental obstacles, and over course time which = money.

For new construction, you want to try and get cheaper infill deals where you can build homes that are unique and have a smaller footprint (less yard space, no pools etc).

More than likely you wouldn't get much of any ROI by adding square footage due to the costs associated with it but if you can find a way to add a bedroom or bathroom to the existing footprint you are more likely to get a better ROI 

If your lender doesn't care whether you close on time or not or doesn't have the experience to handles issues as they come up (and inevitably they do), as @Johnny Wolff said, you could be out significant money.If you're using a credit union or a local bank, they'll want to stay within their footprint and large banks like Bank of America and Wells Fargo are absolutely useless when it comes to investor loans, but there are large lenders and some smaller banks that go everywhere. 

But my crew did it in 8 days and a house with the same footprint/acreage 4 houses down the road sold last week for $230K after it was rehabbed in June/July.

Cannot add or change much to the footprint of condo.