All visa extension questions must be addressed at the municipality in your place of residence in Belgium. For work permits the employer is responsible for seeking the extension, especially for the B visa which allows for a 4 year extension. In general, C permits that are granted for au pairs, domestic workers, agricultural workers and other temporary positions are not renewable. The type A permits, for spouses/long-term partners of EU nationals and people who have lived in Belgium for over 5 years, are initially granted for an unlimited time; if you must re-apply, you will be given instructions when you recieve your visa.
With other visas, the only extensions granted are due to circumstances unforeseeable at the time of the visa application. Upon your arrival in Belgium you need to register with your local municipality within 8 days, and you are issued with a certificate of arrival that states whether your visa may be extended by the municipality or not. Even if not required to register with the municipality, and you are staying in a hotel, youth hostel or campsite, you should register with the municipality where you are staying, in the event you would ever need to extend your stay.
To request more information, contact the municipality where you are staying or the Immigration Services Visa Extension Department
General information on work permits and other visas can also be found on the Belgian government portals: