@Yuri1976
I think "regulated" plots are a lot less hassle than unregulated plots (forests, agricultural land, etc.).
Existing houses are good because they're on regulated land, and they'll have services already connected (water, electricity, septic/sewer). You can also - without applying for permission - knock down a ruin, and build a brand new house on the same footprint.
I have a friend in Plovdiv who imports pre-fabricated houses from Turkey. He does the plans and gets the building permits for the client. I imagine you could get a local architect to do the same for you, if you wanted to build from scratch. He put one up in a village near Plovdiv as his own house, and it looks very swanky. It's a large 2 bed, about 120 m2... and it's coming in at around 90,000 euros (finished and ready to live in).
"Prefabricated" means different things to different people, so it can be a bit confusing and/or seem a bit "low rent". But, usually, the house is built in a factory somewhere, and then the "bits" are transported, and simply assembled on-site (usually on a prepared concrete base). The wall panels might be fully-finished both inside and outside, or there might be some degree of conventional finishing (e.g. cladding on outside, plasterboarding and painting on inside). There are a number of suppliers doing this in Bulgaria, and it seems to be quite a popular option (lower building cost than conventional build). And very convenient for foreigners who don't speak the language, and don't want the time/aggravation of managing a build. My buddy has two ranges: a lower cost which looks a bit prefab... and a more expensive one with very contemporary designs, that looks like traditional construction.
At the simpler end, there are cabins made from SIPs. I made one recently, using 5cm SIPs. It was not as cheap as I expected, coming in at around 16k for a 32 m2 (1 bed) build. And 5cm proved to be a false economy, it's not warm enough and has too much condensation. My village neighbour is a super-practical guy, and he says he would never renovate a Bulgarian house. He loves SIPs! He lays a concrete foundation, builds (welds & bolts) a steel frame, then installs the SIPs. He does this for both agricultural/engineering buildings, and residential. He says it's by far the easiest and cheapest construction method. SIPs have different finishes (e.g. red embossed tile effect is popular for roof), or you can attach decorative cladding.