I think the tax you are reffering to is the equivalent of poll tax, if it is an apartment and you do not own a company to cover this apartment then that is fine, it just means that you dont own the land the apartment is on, that is the same situation I am in.
The reason I say that is to emphasize you are not neeed to pay yearly end company tax return.
The only tax you should have to pay is local municipality tax this is paid in the municipality building in SB or Nessebar / Varna etc and is peanuts, less than 50 quid.
Basically when your apartment is completed you get a bulstat number assigned to your name, this is your number that BG officials tag you and your property against (think of it like a national insurance number for your house)
They say if you do not visit the municipality building within 2 months and pay the tax, then you get a massive fine - thats bull
They do this so you get down there and pay them asap or in my case the management company scaremongered me trying to get me to pay 500 euro to them to do it - I know of over 20 people who have paid late and the biggest fine they got was 20 lev and that was for paying 3 years late