You can easily get to the 7 Rila lakes by car.
The drive is about 1h 30 mins from Sofia city.
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+ no other people on board
+ no fixed departure times
- driving in Bulgaria can be challenging as signs are often in cyrilic alphabet only
- parking near the Rila lakes charlift is difficult during summer season and weekends
- parking fees apply
Getting to Rila lakes by public transport from Sofia city is possible, but quite difficult as there is no direct bus or train.
You would need to take bus or train from Sofia to Dupnitsa town. From there take a taxi up to the 7 Rila lakes chairlift.
Keep in mind that most of the people at the bus and train stations in the smaller cities do not speak English so checking timetables and buying tickets can be challenging at times.