Without seeing a site plan, with setbacks and other building limitations, and the configuration of your house and outdoor spaces, any answer is strictly speculative.

A discussion with your parents is very important and will help identify some of the issues important to them.

Accessibility issues, and family issues such as sleeping arrangements (separately or together), meal preparation and eating preferences, and other lifestyle issues about how the parents prefer to spend day and evening hours will determine the organization and size of the unit. For starters, however, the 800-950 SF should be a good initial target.

The orientation and configuration of that space, however, will depend on the answers to the issues above.

Good luck on your project.

It would help if you post a site plan with what you're proposing and how your house is sited on the lot right now.

Two things you need right away: a survey of your property and a conversation with the planning department to find out all your requirements. Find out what kind of survey the planning department will require. Costs can vary from fairly cheap to quite expensive.

What is an "ADU"?

Accessory Dwelling Unit. I forgot to mention they need to confirm it's allowed in their zone district.

You need permission and architect and you need to ask your parents what they think . If you are living in 1300 sq ft IMO a 900 sq ft unit will looka bit . Have you thought a bout the tiny house movement it might be worth a look. Or adding on to your house which I think would add more value

So that's what I have been designing?

Mark, hopefully you've been designing primary dwelling units too.

Well, I'll jump in to say that I'd rather have my living area and bedroom facing my kids' house than the neighbour's fence. To me that's a no-brainer. Curtains and shades work great for privacy, as does minding our own business which is an inevitable skill that develops when living next to family.

For me and my husband 700sqft would be sufficient as long as it was arranged practically. Right now we have 600sqft serving temprarily as living room and master bedroom. It feels spacious and we have more furniture for our family of nine than one would with only two. We could easily slip a kitchenette in. Add another 100sqft for bathroom facilities. No problem.

Obviously the legal and logistical aspects have to be evaluated on-sight. Theoretically it's possible.

Yes we are working with a professional who has expertise in getting ADU permits. For several reasons, building onto our existing house is not an option. I'm trying to seek a balance between independence/privacy and long-term usability, even after the specific purpose we are building it for ends. Thanks for your thoughts. I especially appreciate hearing that living in 800 square feet is not a problem.

My parents built an ADU and later connected it by breezeway to the main house. My brother and his family moved into the main house. For the first 10 years, it worked well. Later, dementia set in. But, they made it work for as long as they could.

I've got a detached garage apartment above a 3-bay garage. It's 1,092 sq.ft. It has one full bedroom, one full bath, closet, and a very large room with kitchenette. There is plenty of room for one or two people to live very comfortably. I rent it out. Of course, if you are talking about ageing in place, it all has to be on the ground floor. Well planned, 800 to 900 sq.ft. should be plenty.

Design the ADU to meet your parent's needs; that will determine the size. If they require a lot of square footage, perhaps it is not a good idea to build an ADU. Unless, you live in the tiny ADU and your parents live in your home. Locate the ADU where the site and user needs dictate.

For the record I also design primary dwelling units . . . cottages . . . restaurants . . . assisted care living facilities . . . paintball facilities . . . linear accelerator rooms . . . surgical suites . . . . . .

Linear accelerator rooms??

A house near me is one level, 850 square feet. I've been inside, functional kitchen, 1 bath, one bed, small living/sitting room. Very functional if you are a minimalist and have downsized. Actually, I've seen far smaller NYC apartments, but I doubt the latter could be made ADA compliant.

"Linear Accelerator Rooms" = bowling alley? a place for the shuffleboard table? :)

"Linear Accelerator Rooms" = Cancer treatment rooms with four feet of concrete and six inches of lead walls in the main beam area.

Are you sure your county zoning laws will allow you to add an 800 sq. ft. ADU on a lot where the primary unit is 1300 sq. ft? I live in San Jose, which limits ADU size to 1/2 of the size of the primary residence. My lot is relatively large, but my house is only 960 sq. ft, so I'm limited to 475.

My understanding (for LA County) is the 50% size limit applies to attached ADU's. Detached ADU's are limited to 1200 square feet max.