Don't know where you are but here in the US, tiny homes seem to be popular nowadays. The positive is if you move, you can hitch it up and move it with you.

"We would totally be up for sourcing discarded materials, but we don't have any real building experience."

It takes building experience to be able to efficiently reuse discarded and previously used materials. Unless you want to spend the next decade, every vacation and visit, living in a hotel while you are scrounging for materials, buy the plans for a granny flat, with a materials list, and buy the materials. Or buy a kit you assemble yourself.

It may cost more, but the savings on hotel bills has to be accounted for.

I searched Google for this:
australia granny flat diy kits

and came up with a lot of links.

Like this one, using SIPs and shipped as a flat-pack.

http://www.quickbuilthomes.com.au/files/modular-flat-pack-kit-home-the-daintree.php 

And this one, which appears to just unfold onto yout foundation:
http://i-build.com.au/transpack-granny-flats/

And it moves from one part of your lot to another...!

Ooooh, I call dibs on that Victorian Train thingamabob. Dang, Virgil, where do you *find* this stuff?!?!?

To the OP, sometimes it's not the state, but the county (if you're in the U.S.), that has various rules for "accessory dwelling units." Check with them, first. :-)

Modular/prefab houses are pretty quickly built, too.

Just making sure I understand - you have a home in Australia that you rent out while you work abroad but need a place to stay when you are back visiting in Australia so want to build a granny flat in your back yard there? It sounds like what you want is similar to a casita I rented in San Francisco. It was one large open room with 2 queen size beds and 2 futon sofas. One section of wall had a small kitchen with small size appliances. The only interior walls were for the bathroom. It was a simple structure, single gable, relatively low pitch roof and heated and cooled from a single mini split unit. But if you don't have any building experience even a simple structure is a challenge. Expect a big learning curve and longer build time than if you hired it out so you'd renting longer - depends on where you prefer to put your money. Trying to source appropriate used materials will take even longer.

"Just making sure I understand - you have a home in Australia that you rent out while you work abroad but need a place to stay when you are back visiting in Australia so want to build a granny flat in your back yard there?"

Yep, that's the situation.

This, which Lazy_gardens posted seems pretty perfect, but - and I know we don't have any experience - building something ourselves is also quite appealing.

We would never need to spend on hotels as my parents live 15-20 mins from the house and would happily have us. Also we would get the necessary help if we did go down this route. We would probably end up going for one of these plans, but are there any other alternatives which people know of?

Thanks... some food for thought there. I think a caravan would be fine, the problem is I'm not sure if we'd be able to actually get it into the backyard. We had some friends in Belgium who forgot to measure theirs and ended up blocking it in their backyard through the construction (it's not the end of the world of course, losing it).

Yep our kids are young and for sure the 1 room living won't last forever. I guess we will eventually move into the actual house regardless.

I'll definitely look up problems with tiny houses.

Thanks