One is handicapped and will need care all his life, so I would like to purchase a fourplex type building so he can live in one with a carer, and the rents pay for all his costs etc.

Depending on the type of "group home" (drug/alcohol rehab, handicapped, mental illness, elderly, etc.) you have other laws to make sure you comply with, and great point by David D Painter re insurance.

@Michinori Kaneko  There was a podcast - one of the first 120 or so (sorry I dont remember the exact BP podcast # - but a bp member from I think Iowa specialized in renting out his homes to operators of group homes - may have been for folks who were mentally handicapped - cant remember the specifics.Anyways may want to check out that episode and give it a listen. 

Are there any bathroom requirements and/or handicap requirements that need to be met?

Examples: don't adhere to max number guests, rent to inappropriate guests (handicapped person in a walk up was my most memorable!)

One unit was handicap accessible with a really really really long ramp!

I understand Service Animals but they have made getting get an ESA Cert easier than getting a handicap parking pass, anyone can get it.

Of course, we can get into anecdotal arguments about such and such a person who has no choice but to work in a restaurant while trying to support a family, but generally speaking, it’s better to incentive betterment than to handicap them with entitlement.

Some markets handicap you from flips or cash flow type property, be willing to adapt to where you're at 

Our 13 year old daughter is handicapped, so I will always need to have a flexible schedule.