Hi Steven,
That sucks! You are in a bit of a pickle at this point. Always always always research the zoning before buying a property.
Disclaimer: I am not a lawyer, and this is not legal advice. That said, I think I can help orient you in the right direction.
The rules vary from municipality to municipality, but typically if your renovation exceeds a certain percentage of the property's value or square footage, it is no longer considered a "nonconforming use," and becomes illegal. You need to read East Point's code thoroughly to understand a few things:
1) How East Point defines a "nonconforming use" (since every municipality is a little different), and if it is still possible for you to comply with the law and fall under that definition
2) The City's violation and enforcement procedures, so you know what the legal stakes are
3) The zoning regulations that currently apply to the property, and the most expedient procedure to solve your problem. It could be rezoning, or it could be something else entirely, such as a land use permit. You need to know what your end game is, what the approval process is, how long it will take, and what it will cost.
4) Is the zoning conditional? (Usually designated by a "C" tacked onto the end of the district name, for example "R-4-C." If so, you need to call the City and ask for a copy of the zoning conditions that apply IN ADDITION to the general regulations.
5) The property's zoning history, why it was rezoned, and the likelihood that you can successfully obtain the approval you are seeking. You may need to talk with the City's urban planning staff and the City Council member representing your district to determine this. This should be one of our last steps, because you want to have all your facts straight and have an idea of your end goal when you talk to them. Even if you read the code very carefully, they may still provide you with unexpected info at that point, and you need to adapt.
Whether you decide to go for a rezoning or to sell the property as-is, this info will help you know where you stand.
If your property is surrounded by detached houses and there has been a big push to discourage higher-density development in the area, applying for a rezoning may be a waste of time, money, and effort. Be prepared that there is often a lot of unwritten policy in terms of how the code is actually interpreted and enforced. To avoid any nasty surprises, I would give serious thought to hiring a land use attorney who specializes in rezoning cases. The more research you can do and hand-off to them in a clear, organized, format, the more you can save on lawyer fees by decreasing their scope of work.
Again, every municipality is different, but the approval process for a rezoning typically involves an informal meeting with the City's staff, a formal application, and a few public meetings such as a neighborhood committee, a zoning review board, and the City Council meeting. The earlier you can get and respond to feedback, the better. For instance, it is usually good to schedule a non-required, informal meeting with the appropriate City Council member, way in advance of the public meetings. You want those public meetings to be mainly a formality. If you get rejected that late in the process, you have wasted a huge amount of time, effort, and probably money (not just fees but also in holding costs for the property).
Now, the GOOD news (finally, right?)...
It is pretty common to put a property under contract with the closing contingent on rezoning. Either you or the buyer could be the rezoning applicant. It really depends on the goals, preferences, and skills of both the buyer and the seller. Typically you agree in advance to a certain deadline for "the Rezoning Period," so you need to know the rezoning process, what application deadline you can realistically meet, the public meeting dates, etc., before writing the contract. For obvious reasons, you will be able to sell the property for a lot more once the zoning issue is resolved.
Good luck!
Dan
P.S. After rezoning approval, most cities have an appeal period, usually 1 month. During that time an angry neighbor or whoever can apply to overturn your hard-earned rezoning. You can include that time in your contract's Rezoning Period.