If you improve premises you rent rather than own, the work still has a lasting value to your business, so its cost is deducted over years through capital cost allowance (CCA). That is Class 13, leasehold improvements. On Form T2125, Statement of Business or Professional Activities, it is one of the class rows in Area A.
What goes in this class
Improvements you make to property you lease — renovations to a rented shop or office, for example.
What is different about it
Most classes are deducted on a declining-balance basis: a rate applied each year to what is left. Class 13 is not. It is deducted straight-line, spread over the term of the lease.
The calculation is governed by Schedule III of the Income Tax Regulations. It is generally the lesser of one-fifth of the capital cost and the cost spread over the lease period, with minimum and maximum bounds on that period. The half-year rule applies in the year of the improvement.
Dossier does not compute any of this. It keeps the invoice and shows what the class is; the schedule is settled when the return is filed.
What to keep
Keep the contractor's invoice for the work, showing who did it, the date, the premises, and what was done. Keep the lease as well: the term of the lease is what the deduction is spread over, so the lease is as much a part of the evidence as the invoice.
Dossier tags the invoice and files it with the year's records so both documents are together when you file. Don't send them in with your return — keep them in case the Canada Revenue Agency (CRA) asks to see them.
Official CRA sources
Source: Canada Revenue Agency (canada.ca). Not tax advice.