Generally, you can deduct expenses for the business use of a work space in your home as long as:
The work space is your principal place of business; or
You use the space only to earn your business income, and you use it on a regular and ongoing basis to meet your clients, customers, or patients.
Be very realistic and reasonable about how you measure the work space at your home. Claiming 50% of your living space to be your work space could be a bit of a stretch, unless you would like to invite a CRA auditor to inspect your house.
Here is a list of expenses you can claim on T2125:
Mortgage interest (not the full mortgage payment. Principal is not deductible)
Maintenance and repairs
You can’t use business-use-of-home expenses to create or increase a business loss. So if you end up with having more expenses than income for your home business, you will have what the Canada Revenue Agency calls unused Work Space in Home Expenses which you can carry forward into the next year.
The beauty of this is that you don’t necessarily need to claim these expenses in the tax year following either. If your home business continues to meet the conditions for claiming business-use-of-home expenses, “an indefinite carry forward is provided” (CRA) meaning that you can use these unclaimed expenses when it’s convenient to offset higher income in a later year.