Per CRA:
For maintenance costs, it may not be appropriate to use a percentage of
these costs. For example, if the expenses you paid (such as cleaning
materials or paint) were to maintain a part of the house that was not
used as a work space, then you cannot deduct any part of them.
Alternatively, if the expenses you paid were to maintain the work space only, then you may be able to deduct all or most of them.
