Impact FeesSB 13 (2019) removes “impact fees” for ADUs (and JADUs) smaller than 750 square feet and reduces them for ADUs 750 feet or larger to a proportional rate in relation to the square footage of the primary dwelling unit, or average dwelling unit.

SB 13 bans impact fees for units smaller than 750 SF, but for units larger than that it has this text:"Any impact fees charged for an accessory dwelling unit of 750 square feet or more shall be charged proportionately in relation to the square footage of the primary dwelling unit."

It seems to me that anyone who is looking for 3 bedrooms is going to want more square footage than yours has, but I don't know your market. 

@Darren GillespieIf you have the bed / bath count, property square footage and you know the general age and property type you should be able to go onto Zillow and get a general idea.

I'm talking going to see the property (taking pictures, confirming address, confirming square footage), getting an inspection/contractor to access repairs (or perhaps you just figure an estimate yourself), pull comps, & crunch numbers?

We currently have MLS access and can pull Realist Public Records to verify square footage, flood zone, owner name, etc.  

Our current SFH has at least 1 guest bath - guest square footage remains mostly separate from the remainder.

@Jon Mulholland I dont have experience adding that much square footage but I do know that adding at least 1/2 bathroom sometimes requires updating your water supply line (inbound water to home) per Illinois code. 

Is it large enough to fit the minimum square footage house while still meeting the setbacks?

I'm not clear what you mean by "waive" when filing for taxes.You will incur those expenses, but they will be deducted proportionally to rented square footage to arrived at your taxable income.Regardless, I would not consider the tax impact when house hacking.