We considered this, although we found a great 900 sf log cabin (owner built from a kit 6 years ago) and are living in it now, on 25 acres, with plans to add onto the cabin.
Check out www.firstdaycottage.com
- you can definitely get building loans for them. Also www.northeasternlog.com
- this is where our cabin came from, and it was $24k US for the shell and it's 900 sf.
Mortgage Free! by Rob Roy talks about this idea of the "temporary shelter" while you build the big house. His idea is that you live in the temp shelter for a couple of years while building. All from cash. Using cordwood, which is inexpensive to build with (but labor intensive).
The houses would have to be pretty close together to share septic and well.
When we looked at costs, we found that for a $40k, 1400 sf First Day cottage, we'd be looking at $80k in additional costs - that is doing all the labor ourselves except plumbing and pouring the foundation. In general they do say to triple the cost of a kit to figure how much it would cost total to build. In US dollars, where we are, a septic system is $5-15k, a well is $8-10k, and a foundation for a small-ish house depends, but is at least $5k for a slab. It can be much more depending on how much excavating they need to do. Then figure putting in a driveway (even just gravel, or even just dirt!, costs a good bit) and all the site prep (removing trees, leveling ground, etc). There are a lot of costs even with a tiny cabin.
I also don't want to rain on your parade, but we found that the costs are really much greater than we originally thought. We do have a mortgage on our cabin, but it's easier for us to pay down the mortgage with my dh's income (he is a teacher) while we homestead part-time, than to try to "do it all" - this is just what we have found works for us, after years of dreaming and saving!