Unfortunately, the computer mouse and keyboards are considered as non-eligible expenses under CRA. So you cannot deduct these computer accessories as reasonable office expenses. Pens, pencils, pencils, paper clips, stationery, and stamps are some of the expenses that the CRA allows you to deduct.
In the case of capital cost allowance (CCA), they are not typically assets for amortizing over their useful life and are normally deducted over years. Therefore, you cannot claim them as CCA. If you are not sure which items you can claim, you can check the CCA class list here: https://turbotax.intuit.ca/tips/common-depreciable-property-classes-6371
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