Just bought a single family that is in RT-3.5 zoning, wanting to convert to a 2-flat or duplex, is there any reason this isn’t wouldn’t be do-able as it’s already zoned for such? Thanks in advance!

@Laura Ra - There really shouldn't be any reason unless your alderman wasn't okay with increasing density, which did happen to me.  Here is a good resource for Chicago zoning:  https://secondcityzoning.org/z...

I'd also connect with a local architect like @Prashanth Mahakali or @Samuel Pavlovcik , they can guide you through the zoning and permitting process with their eyes closed.

Thanks @Jonathan Klemm for the Mention

@Laura Ra While you are correct that RT-3.5 is designated to allow for 2-unit residences; it is not necessarily guaranteed. The RT-3.5 allows for (1) residential unit per 1,2500 sq.ft. of lot area. This means that in order to have a  2-unit building "By Right" you will need a minimum lot area of 2,500 sq.ft. Assuming your lot isn't irregularly small it shouldn't be an issue; as the standard 25'x125' would easily meet this, and even an undersized 24'x120' lot would be okay. If you have a plat of survey you (or I) can check the lot area and confirm this won't be an issue.

After that, you will still be limited to other concerns including a maximum Floor Area Ratio of 1.05 (on a 3,125 sq.ft. lot you have a buildable area of 3,281 sq.ft. (As long as the basement is >50% below grade it will not be counted towards the FAR. Additionally, you will need to maintain a building height of 35' max (measured to the mid-point of the highest roof), and also required setbacks from property lines.

Assuming your lot is large enough to meet the 1,250 sq.ft. required per unit, you are permitted to add the second unit "By Right" which means the Alderman wouldn't even get involved, and if they wanted to, it would be their responsibility to try to stop you from doing something you are allowed to do "By Right" which would be difficult to do on their end and would likely require legal consultation.(Very, very, very, unlikely to happen)

Assuming we design to meet the Zoning requirements, the rest is all ensuring the design meets building code regarding access, natural light & vent, etc.