The mass penetration of fifth-generation (5G) technology is already a fact. One of the challenges regarding the implementation of 5G networks in Bulgaria is the problem related to the assessment of electromagnetic exposure and determination of safety zones (SZ). Bulgaria has more restrictive national legislation for the protection of public health from exposure to electromagnetic fields (EMF) than the Recommendation 1999/519/EC [1] and ICNIRP guidelines [2]. The first stage of the implementation of 5G undergoes of the upgrading the existing base stations with new installations. This fact raised many questions about the possibility of the maximal permissible values being exceeded. The method in the national legislation for the theoretical calculation of SZ around telecommunication transmitters is based on a conservative approach. It was clear that applying such a method would lead to unrealistically large SZ and will complicate the realization of planned additional 5G emitters on a particular place. The SZ assessment based on the conventional approach was also a complex task for the previous 3G and 4G technologies, but the situation seems more difficult when existing sites must be upgraded with new 5G installations, especially in the urban areas. The presence of different technologies on a certain base station requires the assessment of combined EMF exposure. The specifics of 5G New Radio (NR), characterized by intelligent technologies such as Massive MIMO (Multi-Input Multiple-Output) and beamforming, should also have to be taken into account in this evaluation process. The paper demonstrates the theoretical calculation of the SZ of an existing base station, which is planned to be upgraded with 5G smart antennas. We modified the current method for determination of the SZ boundary around telecommunication sources, which takes into account the specifics of the 5G technology. The application of this method will make the safety evaluation more realistic and the upgrade of existing base stations with the 5G installations to be possible.