Can the city prevent me from building a porch on my home if its on a commerical zone. Other houses on my street have expanded.

This answer is for informational purposes only, does not create an attorney-client relationship, and please note that no specific research has been performed and the law in your jurisdiction may vary. I recommend that you hire an attorney.
Your attorney should look into the status of your house as a prior, non-conforming use. You are correct to be analyzing the setbacks on the surrounding houses. In general, prior non-conforming uses may not be expanded, but they may be maintained and modernized. Your attorney will have to show why this applies to your porch. Also, he (or she!) will contest the determination as not meeting the requirements in your state for zoning decisions to be upheld on review. For instance, decisions cannot be arbitrary. How to prove arbitrariness varies from state to state.
I can't tell from your question whether you have received a final denial. Please be aware of statutes of limitations which may run on your appeal and be sure not to miss the time within which you can file an appeal. Those times can be very short -- just a few weeks or even days. There are also legal principles -- for instance, "res judicata" -- that can make it harder to re-file an application if it has been denied and no appeal was brought.
It sounds to me as if you are being treated unfairly, and it's particularly galling that they would require you to go to the expense of hiring a layer just to be able to get the chance to build a porch.