HABITAT ANSWERS: There are three layers of regulation that govern commercial activity in a condominium: zoning laws, the building's certificate of occupancy, and the condo's bylaws.
Zoning sets the baseline. If the local zoning for your condo prohibits any commercial activity in residential buildings, then that's the end of the story, and you should definitely approach the board with your concerns. Explain that while you have no way of knowing outright, you suspect based on whatever activity you have witnessed.
That said, zoning (and, in turn, the certificate of occupancy) may allow some commercial uses in your condo building — often restricted to ground-floor spaces or certain types of home occupations. For example, it may be that the unit-owner in question is a freelance copy editor or proofreader and is getting a lot of work sent his or her way. That's a very different type of business-related thing than, say, selling merchandise on eBay.
A condo's bylaws usually hew to the zoning rules when it comes to commercial use, but they don't have to. A condo's bylaws can prohibit some or all of the commercial activity that may be allowed under the zoning code.
Even if the law permits a certain commercial use of a condo owner's apartment, the condo bylaws trump the zoning law. If it's against the bylaws, it's not allowed, period.
If you're not sure about the nuances in these rules, then your best bet is to let the board know. You may find that the board already knows about the situation, but if it doesn't, then it's up to the members to obtain evidence of the business. The board may enlist you to keep track of how many people are going in and out of the apartment, or the number of packages being sent and received, and then take it from there.
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