this is also what i would like to build. you can see my stumblings,
ideas and research
here --> greenhouse/cabin.
i am still dreaming of this, maybe can actually work out trying to build this in the next couple of years....i have a couple of good leads on possibly where. looking for the right south facing hillside =) and atm, maybe even a few possibilities.
i have been drawing that again, started over from scratch though. the footings i drew were a bit too large, and i want to try to shrink it smaller, again.
anywho i think you would be interested to research the
PAHS umbrella house ideas, geothermal, and
Subterranean Heating and Cooling System (SHCS).
if it is the costs associated with ready to pour concrete that makes you shy away from it, you could do it considerably cheaper by learning slipform stone masonry, how to make stablilized earth, or by using only minimal amounts of concrete/cement in essential areas, combining them with earthen plaster walls and floor.
and/or with wood, but only for the part not bermed.
light clay straw is one method of making earth walls, which works with post and beam.
if you have a large area of rocks, gravel, sand and clay for free you can work out mixes with your local materials, adding small portions of bag cement and various masonry mixes, so that the costs of the materials is significantly cheaper. in this way you could use what comes out of your hole, and other nearby areas, to then supplement the cement and /or lime /etc...purchased bag materials.
however "free" does not consider the costs of this kind of work for your muscles and back!!! even some small earth mover can make this more realistic.