I used all of my house for day care (except my two teenagers rooms). I have a two level house so the living room, dining room, kitchen and main level wash room are of course used all the time. I also use both my room and the guest room for napping. The guest room is fully for the use by day care so that's the one room which is 100%. The children also use the second level bathroom if someone else is in the main level wash room.
However - I was told that anything over 35% is looked into deeper when you are using your rooms for both business and family. (Different situation when you have dedicated space which is only ever day care).
This is how CRA explain it to me....
If 100% of your house is used for both family and business, then based on their calculation..
100% square footage x 12/24 (they said it's rare an in-home is open longer than 12 hours and so they would look harder at someone claiming to be open longer hours) x 5/7 (again they said it's rare an in-home carer is open on the weekends. So this is the max sq footage x the max expected hours x the usual business days per week = 35%
So someone open 5 days a week using 100% of their house who works 12 hours ends up being able to claim 35% tops.
The CRA agent I spoke with further explained that unless someone was operating out of an apartment or trailer, then it's not actually true that 100% of the space would be used. Built in closets are not used, on-suite bathrooms are rarely used, etc so anything over 35% when there's no dedicated space sends up a red flag.