i started off in a plastering firm doing damp courses, putting steel in the ground with a concrete roof will be asking for trouble, watched the utube vids above and never laughed so much, after a few months of rain, the smell of damp and stuff rooting around the container will be great for a room not to mention condensation mold and a smell to make your eyes water, when the rust starts on the outside and no way to go round and treat it, can see the whole thing lasting 5/10 years max under ground, never seen anything so daft in all my life, why go to the trouble of putting in a steel box when concrete would do the same thing and be a lot more permanent, think more of drainage/ air and light, and less of a metal box, depending on your water table around your house, the soil at that level and at least the same depth below, ie the height of the hole times 2, before it even feasible, for drainage purposes etc
treated concrete would be water proof and insulated enough on its own using ground heat etc, double skin, outer wall with hollow concrete blocks reinforced with bars and in filled with concrete solid concrete blocks on inner walls, tanked with damp proofed membrane, second inner layer as i would also treat out wall before back filled, dpc sheets or membrane and rubber sprayed etc.
as the roof for my build would also be concrete rafts block filled, there would be a choice to add windows or tubes for day lights, this has been done many times in under ground builds and is a proven method of getting light into a underground dwelling with out much impact on the surface. ie not seen etc.
when making the hole i'd also put in drainage, water pump, and electric unit piped from the main house, using a pipe means if anything goes wrong you can always add more down the pipe etc or replace.
as most users can lay concrete blocks, and the cost is very low for concrete, around 10k would be easy to achieve on a self build, just remember the spoil heap will need about same amount of space on the garden as the size of the hole, so you would need to leave plenty of garden left for spoil only, not shown on your plans.
where as some can be used to in fill and cover the build, 10 /20 tonnes plus would need to be costed in to take away if your garden couldn't take the extra, ie be built up etc
1 tonne of spoil is not even an inch over a 10x10 ft plot but mounts up if you have less area to use left etc, so depends on plot free and ground level etc, if garden slopes, you could build a retaining wall and make the whole garden level, would be cheaper than taking it away etc
h88p://www.ehow.co.uk/how_6207369_build-underground-concrete-shelter.html
h88p://www.greenhomebuilding.com/QandA/earthshelter/how.htm
h88p://www.concreteconstruction.net/images/building%20underground_tcm45-347108.pdf
Planned it out too!!
