There are many sites with designs for small houses. I'll link to some later but others here will know some. There is a GardenWed forum for small houses too.

The issue of legality should not be a concern. The minimum standards of building codes are easy to meet and you will want to exceed them for some elements. Online designs don't always show all of the code controlled elements anyway.

What is required for a permit varies greatly with jurisdictions.

http://www.countryplans.com

What do you mean by barely legal. Are you thinking of forgoing running water like in a hunting cabin or something? Or are you just talking about sq ft minimums? Define your terms a bit more.

The tumbleweed tiny house plans company has some small foundation built houses.

Freegreen and Katrina cottages another designer with small budget oriented plans.

You can also search houseplans.com for small houses and get a few viable options.

I don't know anything about concepthomes.com but they have a category of less than $100,000 to build homes.

Find out what the minimum for permitting is in your area and then look for that.

There is a good forum at countryplans.com, with a lot of like-minded people.

Thanks all. I'll study your suggestions so far.

To answer DLM2000's question, by "barely legal", I meant fully functional and passing code requirements for human occupation, but zero fanciness. I was assuming that these days in U.S. that would include a functional kitchen and bathroom. (is that assumption incorrect?)

Basically just something that is a legal residence and which can then be embellished slowly over time. (and which is inexpensive enough it can conceivably be paid for out of pocket during rather slow largely DIY construction cycle.)

It really depends upon what the local codes are. Some parts of the country have no zoning or inspection; others are quite draconian. Generally, there are code minimum sizes for a dwelling and individual rooms, you have to have a functional bathroom and kitchen, septic system (those are a biggie pretty much everywhere) sufficient heat to maintain 68 degrees, and whatever NEC Code they enforce. Electric (NEC) codes are getting much stricter, too. In most places they now require tamper resistant outlets and arc-fault breakers, on at least part of the house. Believe it or not, some states now require sprinkler systems in houses. Most places require that the outside of the structure be 'finished', which usually means at least paint. Energy codes are all over the place, from almost nothing up to requiring blower door testing to check for air leakage. I read that in CA, they'll require new homes to be 'net zero' by 2020, which means they create as much energy as they use on an average. As you can see, a guy could live in a tool shed in some locales, and have to have a state-of-the-art home in another. The first step is to research exactly what rules you will have to follow.

Building codes are the minimum needed to ensure comfort and safety in a home, and yes, there are national codes that are applicable, even if they lack local enforcement. Nothing says that you have to build to the lowest requirement though. In fact, as far as energy efficiency and air tightness goes, you'd be shooting yourself in your future foot if you built to minimums.

Start with the amount of basic space that you would think that you would be comfortable with living in for 2 adults, whether or not there are two adults. Then design a small home based on that. Design it from the beginning with room for future expansions of a kids bedroom wing, maybe a master wing, or a family room addition across the back. It will cost more later to do all of that as additions than up front construction, but it lets you add things as you can afford them, IF you design the plan as a whole from the beginning. That means taking into account site conditions, roof and foundation structures, and plans for HVAC for all of the future space. It's 3x as much planning to be able to do it that way, but it's the only way to successfully do what you want and not have it end up being an awkward hodgepodge warren of wierd spaces.

Sophie, you are correct about one thing here...planning ahead and choosing a house plan that lends itself to expansion is both wise and important for a plan like mine.

The amount of time it takes for an energy-related choice to pay itself back is also something to consider carefully. (...while keeping in mind that the technologies/costs involved are changing more and more rapidly)

Keep in mind though that the comfort level and safety level prescribed by the codes is by definition wrong for many people. Comfortable enough varies from person to person, area to area, century to century.

Safe enough has to be taken as a gestalt. If one has limited means, is it safer and more comfortable to live in downtown Detroit in a code-compliant hovel...or in a carefully built shack in the woods?

In an increasingly unstable civilization, how risky is it to choose a big mortgage over a little "discomfort"? Each person has to make these choices for themselves (to the limited extent allowed by the law).

...and no, I'm not a "prepper"...very far from it. :)

Being watertight, warm, and not frying yourself from electrical wierdness or ending up in the hospital due to mold growing in your house IS the basics that the code covers. There really isn't anything not related to those basics that's been codified. And always as a response to some disaster or loss of life.

You have about 3 years of planning ahead of you, minimum. Start off by a trip through Amazon's book section and buy anything Amy Johnston and Sarah Susanka have written. Then go do some construction volunteer work for H4H and hang out with the pros running the job sites.

Sophie, I guess you and I will have to agree to disagree on how awesome the codes are. As for your other suggestions...the volunteer work strikes me as your best advice.