I think the information about the ADU process is already available. Now that everyone is excited about ADU's, lets talk details. The below is specifically geared toward backyard ADUs. If you are in the pilot area and become eligible to build an ADU over the garage and be the first of two in your block, read on:
1. Foundation: You are in the pilot area and decide to do this. At a minimum, the "dwelling unit" that you want to situate over the "garage" would now put more loads on the foundation which, in most cases is a thickened slab.
2. Water service. As of now, the water service comes to the front of the building and goes into the meter. For the water supply to reach the rear ADU, you would have to either come through the house or outside of it with its own meter.
3. Separate electrical service and panel? Typically there is one service feeder to the building. In the case for a backyard ADU, would there be another service drop?
4. HVAC: Need to insulate and drywall the garage ceiling now potentially with more sheets of drywall at the ceiling since now someone is going to reside over a fuel-burning device: Car. Also, the shell needs to be completely tight in order to prevent any gases from sucked into the unit above.
5. Noise: every time that garage door opens, someone is going to wake up angry! so lots of insulation.
Now, let's talk about the basement ADUs which are more feasible but with fewer issues. Most of the issues related to backyard ADUs above such as foundation, HVAC, Noise, and utility separations etc. are easier to tackle in a basement ADU. Although some issues to think about are:
1. Water service: Most of the homes that want the basement ADU should come with a water service upgrade waiver. The city of Chicago used to have a form to apply for the waiver. If building department forces you to upgrade the existing water service, that could be an expensive non-starter.
2. Ceiling height: You may be eligible to create your ADU with less ceiling height but it wont be a desirable or healthy space. However, if you dont meet the ceiling height requirement, you would have to lower the slab. This is a major structural undertaking depending on how far your current foundation extends below the basement slab. Avoid underpinning at all costs.
3. Water control: At a bare minimum, before you make a current vacant basement into a dwelling unit, make sure to have a drain tile connected to the sump pump and an overhead sewer system. WIthout these, most of the ADUs are going to get a bad rap a few years from now.
In summary, while I am happy that the city has taken up this initiative, it's severely lacking in many fronts and my fear is that it will not achieve the goal: Creating more affordable housing stock. Rather, it will create issues for people who cant afford to add these ADUs but are going to go for it anyway, or the building department is going to make it difficult by enforcing building code requirements. Next time you get to talk to a building inspector, ask them what they think about this. You will get the real run down! Sorry for sounding cynical but this is a step in the right direction but we have ways to go!