PUBLIC TRANSPORT TO / FROM THE APARTMENT
TAXI FROM / TO THE AIRPORT:

https://www.sofia-airport.bg/en/passengers/and-airport/private-transport

Taxi would usually take 15 - 20 min to the apartment, depending on traffic and would normally cost up to ~20 BGN (~10.22 EUR)
METRO FROM / TO THE AIRPORT:
https://www.sofia-airport.bg/en/passengers/and-airport/public-transport
​
There is a metro / subway station right near the apartment (~2 min walk), called Opalchenska Metro Station and the train usually takes less than 30 min to get from/to the airport:

​2 min walking:
"Opalchenska" Metro Station
MAP

Metro single ticket cost is 1.60 BGN (~0.82 EUR)

Sofia Metro:
https://www.metropolitan.bg/en/scheme/operating-metro
COUNTRY BUSSES: 
https://www.centralnaavtogara.bg/index.php#b  

22 min walking / 8 min by car:
Central Bus Station - Sofia 
bulevard "Knyaginya Maria Luiza" 100, 1202 Tsentralna, Sofia 
Tel: +359 2 90 021 000 
MAP