Check with your local municipality first as to what will be allowed. A secondary dwelling unit is not allowed in many many locations. Then, if allowed, you will need to operate under the rules that do allow them. Many do not allow kitchens, etc.

If everything aligns correctly, the largest obstacle then will be the plumbing. That is when you get an experienced plumber and builder in to address the limitations that adding plumbing will have on the design.

You are way ahead of yourself. You don't know the constraints yet.

Good advice from Sophie. From the residenr's perspective, I think it would be good to design as a big suite, where it'd be easy to move around in case of wheelchair, no hallways, least amount of walls. Again, I think a layout like a motel suite

Well, as an old lady, I'd like a nice living room with bookshelves, a small kitchen (galley style), a dining area for a table and chairs, a bath with both A tub and a separate shower, a bedroom big enough for a queen size bed, chest and chaise langue, and a large walkin closet. A storage closet and coat closet would also be nice.

I second Anglophilia.

Have you asked the relative what s/he would like? For me, front door in the kitchen would not be ideal. Kitchen appliances could be small, as they're just for one person. A stackable washer and dryer could go in the bathroom -- or even in the walk-in closet. A vacuum and cleaning supply closet would be a real plus.

How much will the relative be bringing from his or her former home? Any furniture? Will there need to be additional storage for keepsakes or hobbies?