@chrisdrobson
Welcome to the expat.com forum and good luck with your Bulgarian adventure!
As others here have said, it's probably a lot easier to get an already-regulated plot. There are also many large ruined houses sitting on nice plots of land, and these plots are regulated, and have (usually) water and electricity already connected.
In many villages, things are pretty laissez-faire, so if you had a house, I doubt that anyone would kick up a fuss if you simply plonked a little cabin in the garden. But, officially, you have to get planning/building permissions, just like the UK, unless you put it on the same footprint as the existing house (or barn or other outbuilding).
There are plenty of suppliers of prefabricated cabins and houses here, but it can be tricky to track them down. Or you can build your own.
I've seen several TopHouse (dot bg) billboards next to the A1 motorway. I have a friend in Plovdiv and he has been running a very successful prefab biz for several years. You can see him (and his catalogue) in central Plovdiv (and online at homeskisiov dot com). He imports them from Turkey (by road), and then assembles them on-site. Turkey is a very low-cost supplier of construction materials to Bulgaria (due to their currency problems and its proximity).
I don't recall seeing an A-frame cabin type, it's mostly small houses. He has two ranges: a budget option and a fancy modern one. The basic one is good, but looks a little too prefab for my taste. I saw the fancy one recently as he built one for his own home, and it's very nice. In his case, he bought a ruined house in a the centre of a very nice village just outside Plovdiv, and then cleared the plot.
Our house is in a lovely village in the Balkan Mountains, so I've seen quite a lot of local activity (I'm a bit of a nosey parker, innit). Most houses have been renovated, rather than knocked down. I was quite surprised by a couple of them as I thought they were pretty far gone, but the owners managed to save them. But there are plenty of examples where they knocked down the old house to build a fancy new house (usually much bigger than the existing house). Quite a few have squeezed a 2nd new house onto their existing (larger) plot. There are also quite a few houses built on plots at the edge of the village (i.e. regulated, but no house yet).
One of these folks put up a beautiful prefab log cabin home, I was there while the trucks delivered the wood and panels. It's certainly a fast way to build, and it looks like a great house. I don't know if these guys (loghomesbulgaria dot com) supplied it, as they are more log-y, but it's this kinda wooden thing. :-)
One of my neighbours is the well-driller for the whole village, and many villages in the region, so he seems to be a very switched-on guy. I got him some parts for his old Land Rover off Ebay a few years back, so he is very nice to me and always shows me what he's working on. :-)
He recently built a family home nearby. He also went the route of an existing house in the centre of the village, which he knocked down and cleared. He self-built a large (240 m2 give-or-take) single-floor prefab. He laid a simple large rectangular concrete base. Then he put up a steel-beamed support structure and fastened standard SIPs. It's still recognizably a prefab box, but it looks good as he put some nice cladding on the outside, and a very nice black flat roof. A few tiny windows at back and sides, but 6 large windows at the front. On the roof, he has a solar water heater and about 16 solar panels. He used his well-drilling machine to dig a deep borehole in the garden for his heat pump which provides underfloor heating. Overall, very well-insulated and eco, I'm sure it's pennies to keep it toasty.
I even dabbled recently, so I am the proud owner of a small (24 m2) prefab cabin. Nowhere near as cheap as you think it's gonna be, I've been horrified by the total cost. And it still looks like a cheap container house. :-)
More generally, I've found that many of my construction/renovation/furnishing costs are a shocker here. It's easy to get the wrong idea when you see really inexpensive properties for sale (see "we bought a village" and various Ebay adventures). But labour and materials are much more expensive now, so anything you renovate, knock down, or replace will cost a bunch of money. I'm still a huge fan, and I think you can still end up with something amazing when you compare like-for-like with the UK. But if you buy an old house (or empty plot) for 5-10k, you won't spend another 5-10k on it... you'll probably spend 40-50k (and up).
Which is not to say don't do it! But just not to get over-excited, and try to have a more realistic view of your eventual costs, and the aggravation of undertaking these projects in a new country with a language barrier. And perhaps to give weight to properties where some of the work is already done (e.g. plot is already regulated and cleared and has utilities, or it has a nice private perimeter wall already built), or it has a decent house you can live in, and renovate, while building a new place on your large plot. In light of the expected TOTAL cost, I'd also suggest it makes sense to spend a bit more on the initial purchase so that you're in a nice village, in a scenic region, with relatively easy access to supermarkets and other services.
Well-driller house
fancy wooden cabin
Kisiov village house (not finished)
My prefab