Most building access systems tend to use 125KHz unencrypted HID cards such as a Proxcard II.
These can be cloned trivially to any writable T5577 tag via a professional RFID analysis tool like a Proxmark 3 or even a cheap generic “HID cloner” off Ebay.
Newer HID cards are based on the Desfire EV1 which are theoretically open to side channel attacks that would take several hours but no specific attacks have been published so far.
Other buildings often support 13.56MHz MiFare NFC cards with no encryption. A smartphone can easily clone these to most NFC tags, or a NFC enabled Security Token like a YubiKey Neo.