Advice on building a 3rd unit on RT 3.5

[Isabel Rao]

Hello All: 

I am a new investor, considering purchasing a 2 unit multifamily in an RT 3.5 zone. The lot is standard 25 x 125 so I cannot build a 3rd unit in the basement "by right". I read that an ADU might be allowed if it is accesible for people with disability. is this true? What would be the process to appeal to alderman to allow for a 3rd unit?

thanks in advance