I've thought about this a ton. My family is me, DH, DSS14, DD6, and DS3.
My first idea when we were thinking of building (which we still entertain, but right now we are leaning toward buying an existing house with acreage) was to build an earth sheltered home. I still really like that idea, but it has two problems, one being the cost/financing, and the other being the fact that in order to build a house like that, you really have to find the perfect spot- hill or bluff, southern exposure, appropriate soil, etc.
Also, when we were first exploring the building process, I came to realize that building codes/permitting where I live are insane. My dad was contractor for 25 years and he concurs. We pretty much couldn't do anything we wanted. So I came up with some alternative strategies.
First off, I'd buy one of those ready made sheds, about 12'x12'. I'd buy it because those are permitted as deliveries county wide. Building one would require a permit. I'd insulate the heck out of it, and, depending on the lay of the land, put either a trap door in the floor or a door in the rear. I'd dig out a root cellar under or behind my cool pantry, and put a few solar panels on/around it, so that I could have a fan if needed, lights in the root cellar, as well as a deep freezer and chest fridge inside the cool pantry. Best I can figure, I'd spend close to $3000 on this, including renting the mini-excavator, shed+delivery+insulating supplies, hardware, and a small DC solar electric system. Which is a lot, I admit, though not much in the scope of a house.
Then I would go ahead and get a permit and build a very well insulated "two car detached garage". If I snared my dad into this project, it would cost about $10,000 (not including PV electricity). After I had passed inspection, along the south side I would install laundry machines, build a porch for drying laundry, and, in the southwest corner, put in a clawfoot bathtub in it's own stall, a composting toilet with sink in a separate stall, and have a stall with just a mirror in it for changing. Along the north side, there would be four standard (2' deep x 4' wide) closets. Each closet would be tricked out- one for sewing, one for art, one for office, and one for fly tying. Down the center of the building would be massive shelving and rods for storing clothing and shoes. There would be a drop down ladder into the attic, some of which would be seasonal storage, but mostly finished into a playroom for the kiddos. The whole thing would ideally be covered in photovoltaic panels and solar space heaters/in floor radiant heat, though I'd probably have it hooked up to propane for back up heat (needing propane to heat water and run the dryer), though I might be able to pull it off with some sort of outdoor wood fired boiler/hot water heater, with a solar hot water heater for summer. I'd like to just have it on a point well, since DH can drive one. (He's also the plumbing/gas hookup guy. I'm the electrician.) I'm not sure yet if I want to put the outdoor/canning kitchen along the east side of the "garage" or just have it as a separate facility under some type of pavilion.
As for our house, we would either buy or build something along the lines of a park model RV, depending on financing. No permit required, because it's a camper- any place where you can park your own RV (virtually everywhere here) you can put a park model. Cost would be about $30-$40K. Super insulated, of course. ~400 square feet for the main floor, consisting of a bedroom, 3/4 bath (composting toilet, natch), kitchen (wood cookstove/heat source) and dining/living area. No stairs, just ladders up to the split loft. One loft per young child. Bedrooms will have small closets/cabinets that can hold 3-5 days worth of clothes and shoes and no junk. Then I'd like to build a greenhouse that is about 8'x20', connecting the side door of the park model to the side door of the "garage". 2' deep counters/shelves along each side, with a walkway made of pavers down the center hallway. Pavers would heated by in floor radiant heating and an EcoTemp that lived inside an insulated box outside the greenhouse, probably with its own 100# propane tank. It'd be dead simple to fix if something went wrong, since the pavers wouldn't be in cement, just sitting on a gravel bed. Again, I'd like to run it on PVs (another $3000) and a point well.
Since DSS is going to be grown before we know it, I'd like to involve him in building his own place, probably something like The Little House Family's 8'x21' house on a trailer. You can move an 8' wide trailer without a special permit, and who knows where a young man will end up. It'd be nice, I think, to be 18-22 and have your house on your back like a turtle. This would cost between $10-$12K for the DIY version, $2000 alone going for the trailer.
The beauty of this is that when SIL and BIL retire to our area, they can build their own tiny house near us and this infrastructure will be extremely flexible, allowing them to build only exactly what they need. (No kids, just the two of them, and we already have a scheme in place for them to build a movable house and pay us "lot rent".) If DSS wants to stick around, he could, while having his own space and responsibilities. I'd like to build a tiny, movable house for each of the younger two, as well, when the time comes.
Since this all a fantasy, we shall also include: a machine shed (a place to store/work on vehicles), a chicken coop, 1-2 small livestock barns, a solar shower, an outhouse, a small garden shed, a bunch of hoop houses, and a giant composting structure. And maybe a screen porch.
The advantages would be that we could do this a step at a time, it's easier to heat and power small buildings, the taxes would be low, and the permitting would be far less painful.
Since we just got vacant land financing in order, this might happen if we find raw land that we like before we find an existing house.
