I'd speak to an engineer first.

I am researching this also so interested in replies.

Even though I love the look of a container and they have a lot going for them, the dollar's just didn't add up for them to be a cost effective solution when compared to standard building practice's. They are just a bit to narrow which is where the main issue stemmed from. Joining two together work's but again paying a metal shop to do it bumped up the cost's with structural steelwork, engineering detailing/shop drawing's, etc. I used to be a boilermaker welder many year's ago so know what was needed....... Shame they didn't work out cost wise as I can see the attraction. Nevertheless, I would investigate both option's of using a container as the starting point, then compare it to if you used standard building processes of timber frame and cladding with the same dimension's......

what is the ground/floor made out of, what's the height of the whole space. is the ceiling (bottom of the upstairs framing and floor) the same height throughout the whole space, are the stairs inside or outside and do you need internal stairs from the new rooms to upstairs or a separate external entry, where is the street frontage, is there access around both sides of the house and is the view at the back or front of the house etc?...we can make suggestions for possible floorplans but you may need Council approval to enclose the space under the house for liveable rooms so suggest you first consult them to make sure that what you want to do is legal and possible but there are many questions to be answered and best answered with an on site inspection by a professional before you can make any serious plans but doodling with the space shows that there's plenty of room for quite spacious self contained accommodation