It really depends on the zoning in the area. If you go to your city, you should be able to find out if businesses are allowed in that area. If not, then it may be against the law. http://www.ghalyhwc.com/index.php/services/
If you wish to rent an apartment where you are able to conduct business, have clients and generate income, you will have to rent in a work/live apartment or in a mixed use building. the use of the apartment is usually advertised in the description of the apartment you wish to lease. Most of the time, you will not be able to rent an apartment and turn it into an income producing business unless you lease a commercial unit or a work/live unit. The units have to be zoned as such.
I agree with Mr. Park's response. It is not without precedence in Manhattan or elsewhere and you oftern see Doctor shingles or other signage on buildings as well. Yes, check with management if this is really something you feel a need to follow up on.