With that said, here's what you should bring to talk to the counter person at Planning:
The address of the property, the assessor parcel number or map. They will give you the current zoning information including allowed uses and those that require a conditional use permit. If none of these uses are inadequate for what you plan to do, then you should ask to rezone the property.
To rezone a property usually requires you to put together a proposal stating why you want to do this, and a site plan of the property showing the proposed new building. You do not have to have a complete design at this stage, but you should still consider hiring an architect to take you through the process. You will have to fill out an application, pay a fee, notify neighbors (usually up to 300 feet from the property) and you will likely be required to participate in a public hearing. This process can take several months.
To find a local architect, contact your local chapter of the American Institute of Architects (AIA).
If you are keeping the house and have a specific business tenant in mind the variance might be the way to go.
That way you have the best of both worlds - if the advantage of business use evaporated in the future and res use was preferable financially, you would not have to go through a full re-zone again. Another thought is to get the variance for a mixed use or live / work space.
The full re-zone might be have higher expenses and requirements than a variance.
I'd like to see the property pitch in an email when you write it.