@tim1
Sorry, I stand by my post above.
Care should be taken.
See ITA S45(1)(b) (Strictly: ANY part used for some other purpose)
Administratively, CRA are slightly more generous than that (See T4037 (etc))
“However, you are not considered to have changed its use if all of the following conditions apply: (1) your **rental or business use of the property is relatively small in relation to its use as your principal residence (2)… etc”
…“If only a part of your home qualifies as your principal residence and you used the other part to earn or produce income, you have to split the selling price and the adjusted cost base between the part you used for your principal residence and the part you used for other purposes (for example, rental or business).”
CRA used to use the administrative words “Incidental”. Now it seems they prefer “relatively small”
In any event, the person referred to above is certainly not using the whole house “solely as a principal residence, except for renting out a small guest room to an exchange student for a school term”.