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In an earthquake zone? That's too funny.

Actually, correctly designed and built underground houses are considered safer than above ground houses in earthquake zones, even in high risk zones.  Whether someone could get one officially approved in Bulgaria is another matter. I don't have any experience of applying for building permits, but hear it's quite a process even for a conventional home or extension.

As far as I know, it's allowed to build a new house on the footprint of an older house without needing a permit. The partially underground cellar is often all that remains of ruined houses, and can legally be built on, which possibly could include just giving it a roof. But that would be a very small house -- just the single room. My understanding is that extending the footprint needs a building permit.

You may do better posting the question in a natural building forum or Facebook group. Many of us here have traditional earth built houses but no one has mentioned a new-build underground house.

@Mic Yarden

Sounds like a fun project, if you're thinking of a bit of DIY. :-)

My guess is that if you engage a Bulgarian architect to polish your plans and deal with the relevant authorities, you could probably get permission for pretty much anything.

Alternatively, given how laissez-faire they seem to be out in the sticks, you might be tempted to buy an old village house with a large plot of land, and just build your secret project in the garden on the QT. If it's an earthhouse and off-grid, how would anyone even know it was there? :-) If you drill a new borehole for water and put up 5 kW of solar panels, I doubt anyone would care.

In which case, it would be better if you can find one with a nice stone border wall (very typical) for privacy. An existing house gives you somewhere to camp while building your dream underground bunker (or chuck a caravan on the land instead). Moreover, an existing house will have electricity and water in place which will be very useful from a construction perspective. As will having a secure place to store all your tools and toys. Both electricity and water are relatively inexpensive here, so the financial advantages of being off-grid are not so significant. If you do go off-grid, you can still keep the mains services as backup as there are usually no standing charges here (only for actual usage).

Around my way (in the Balkan Mountains near Kazanlak), I've come across a number of properties that are significantly outside the village and are definitely off-grid. And pretty sure they would never have gotten a formal permission for building in such a location. This seems much riskier to me, but if you can find a very large and very wild plot (maybe forest?) somewhere that's not highly trafficked, that might work too. Probably they were built way back in the "good old days" and perhaps enforcement is a lot more serious now. But... I also see plenty of foreign-registered vehicles used as village runaround with (pretty sure) no tax, MOT or insurance, and a bunch of trucks who do illegal logging in my mountain forest. If the cops and forest rangers don't visit, probably not many town planners wandering around looking for illegal construction either. :-)